These components make rather minor alterations to the spells in the game, usually in the interest of game balance or consistency. All changes are fully documented in the spell descriptions themselves. Note that while all these components are optional, many of the tactical choices made by wizards and priests in SCS assume that they are installed, so you may notice some slight anomalies in enemy behaviour if you don't install them all.
Standardise spells between BG and BG2 (TUTU,BGT)
At the moment, BG1Tutu and BGT are both a bit inconsistent in their use of BG2-only spells (like Stoneskin or Holy Smite). Non-player characters don't have any of these spells, and enemies don't use them, but party sorcerors, clerics and druids have access to them, and party mages can choose them at first level.
This component offers two alternative ways to resolve the inconsistency. The first (available only for BG1Tutu players) blocks player access to all BG2-only spells except for Find Familiar and the various wild-magic spells.
The second alternative reintroduces into BG all of the new wizard spells of fifth level or lower. Some are added to stores, others to treasure caches; see here for all of the locations.
Note that this is not needed for BG:EE because the BG2 spells are already available. (In fact, the code to make them available is based on SCS code, so in effect this component is already included in BG:EE.)
Make Protection from Normal Missiles affect some magical projectiles
This component improves the power of the Protection from Normal Missiles (PNM) spell. It now blocks most magical arrows, bolts etc that lack a formal "plus": that is, it blocks fire arrows, ice arrows and the like. The exceptions are the Arrows of Detonation, the Arrows of Slaying, and the Arrows of Dispelling. Combined with the component that replaces +1 weapons with nonmagical weapons, this significantly increases the usefulness of PNM (read: makes PNM not entirely useless for NPC mages).
Allow Spellstrike to take down a Protection from Magic scroll (BG2,BGEE,BGT)In the original game, the protective sphere produced by a Protection from Magic scroll is completely impenetrable by any spell at all - if you use one, all a hostile wizard can do is run away or fight in melee. This component allows the most powerful mage anti-magic spell, the 9th level spell Spellstrike, to destroy the anti-magic sphere of one of these scrolls. It also allows the anti-magic ray of the Hive Mother beholder to destroy the sphere.
More consistent Breach spell (always affects liches and rakshasas; doesn't penetrate Spell Turning)
Although it isn't documented, the 5th level spell Breach will remove a creature's combat protections (such as Stoneskin) even if that creature is protected by Spell Deflection, Spell Turning or Spell Trap; it will not, however, affect creatures like liches or rakshasas, because they are immune to spells of level 5 or below. This component removes both features: Breach now bounces off Spell Turning (etc.), but it affects even those creatures immune to "normal" 5th level spells.
Once this component is installed, the Breach effect of the Wand of Spell Striking will behave in exactly the same way as the Breach spell.
Enemy wizards will assume Breach works this way (and so won't target characters protected by Spell Turning etc with a Breach), even if you don't install this component.
Antimagic attacks penetrate improved invisibility
This component changes those spells which target a creature's spell defences (e.g. Secret Word, Spellstrike, Ruby Ray of Reversal - but not Breach) so as to allow them to target creatures which are protected by Improved Invisibility. The idea of this component is that these spells can now be cast on enemy mages who are protected by Improved Invisibility - this makes the Improved Invisibility / Spell Immunity: Divination combination less overpowering.
Prior to version 16, this component worked by giving the spells a small area of effect. This option remains, as a legacy component, for those who prefer it.
The antimagic attacks of enemy mages will bypass Improved Invisibility even if you don't install this component.
Iron Skins behaves like Stoneskin (can be brought down by Breach)
For some reason the Iron Skins spell, although it appears identical to a Stoneskin, counts as a Spell Protection (like Spell Turning) and not as a Combat Protection (like Stoneskin). This means that in the unmodded game, Breach does not bring down Iron Skins, but (e.g.) Secret Word does. This component relabels Iron Skins as a Combat Protection.
Enemy mages will assume that Iron Skins works this way (and so will, e.g., cast Breach at it) even if you don't install this component.
Modify the Harm spell so it does damage rather than reducing target to 1 hp (BG2,BGT)
The Harm spell reduces its target to 1 hp in 2nd edition D&D; in 3rd edition, it does a large amount of damage instead. This component modifies Harm to work more like the 3rd edition spell: it inflicts 150 hp damage, with no saving throw, on a successful touch attack. The rationale for this is that Harm (especially combined with Critical Strike) is otherwise an almost-instant way of eliminating even foes with hundreds of hit points. Harm remains lethal in this component, but is not quite so overwhelming.
There are two versions of this component. One version modifies only the player's version of Harm; the other modifies player and enemy versions alike. (The former component is probably more merciful, as rather few characters in the party will have more than 150 hp even at full strength, at least until very late in the game.)
Make individual versions of Spell Immunity available, so that players can use them in Contingencies etc. (BG2,BGEE,BGT)
This component is self explanatory. It should be irrelevant for the vast majority of old-BG2 users; I leave it in place for anyone who for some reason isn't using ToBEx.
Revert Greater Restoration back to only affecting one creature (BG2,BGT)
Throne of Bhaal makes an undocumented (but widely known) modification to the Greater Restoration spell, making it into an area-effect spell that heals the entire party. This component reverts Greater Restoration to its SoA version (also its BGEE version), where only one creature can be affected.
Enemy priests assume Greater Restoration affects one target only (and so fairly rarely use it) even if you don't install this component.
Blade Barrier and Globe of Blades only affect hostile creatures (BG2,BGT)
Even in the unmodded game, hostile priests used a special form of Blade Barrier that only affected their foes. (This is probably because, in the Infinity Engine, it is almost impossible to script creatures to avoid them crashing into each other.) This component (again in the name of fairness, and also of hassle avoidance) extends this to the player's version of these two spells: with it installed, players and NPCs use the same form of Blade Barrier. (They don't quite use the old NPC version, which had a rather shorter range: they use a hybrid version, as large as the old PC version but affecting only enemies).
Cap damage done by Skull Trap at 12d6(BGEE,BG2,BGT)
Skull Trap, like the other L3 spells Fireball and Lightning Bolt, does 1d6 damage per level. Unlike those spells, though (and, I suspect, via an oversight on the developers' part, though I could be wrong) Skull Trap's damage is not capped at 10d6, which makes it extremely lethal for a third level spell by the later part of the game. This component caps the damage of Skull Trap at 12d6 (the slightly higher cap reflects the much shorter range of Skull Trap compared to Fireball.
I don't know whether this component makes the game net easier or harder. I was led to write it after seeing a lich in SCS II playtesting cast a 3xSkull Trap Spell Sequencer, doing about 100 hp damage even to characters who made all their saving throws.
Make Power Word: Blind a single-target spell (BG2,BG2EE,BGT)Unlike other Power Words, Power Word: Blind has a small area of effect. This both makes it rather powerful, and makes it virtually impossible for enemy AI to use without "friendly-fire" incidents. This component just makes Power Word: Blind target a single creature. (This is another component which the mage AI will assume is installed.)
Make Minute Meteors into +2 weapons (BG2,BG2EE,BGT)
In the unmodded game, the meteors created by the third-level Melf's Minute Meteors spell strike as +6 weapons, enough to penetrate any creature's resistances and to cut through even the Absolute Immunity spell. This strikes me as a bit overpowered for a third-level spell. This component makes meteors +2 - still enough to hit most creatures, but not enough to cut through any of the protection-from-magic-weapon spells.
Reduce the power of Inquisitors' Dispel Magic
Inquisitors have a very fast powerful Dispel Magic ability, which is cast at twice their level. In most bits of the game, this is usually much higher than the majority of spellcasting enemies. This component reduces the power of the Dispel a little; you can choose to use it at 1.5xlevel or at 1xlevel.
Slightly reduce the power of Insect Plague spells (and the like) and let Fire Shields block them
In the unmodified game, Insect Plague and Creeping Doom are very hard indeed for a spellcaster to deal with or defend against. This component (which was partly inspired by Demivrgvs's Spell Revisions) grants a per-round saving throw against the spell-failure effects of the insects; it also cause the insects to be destroyed if the target protects himself/herself with a Fire Shield spell (either colour) or Aura of Flaming Death spell.
Cosmetic change: stop Stoneskins from changing the caster's colour (BG2,TUTU,BGT)The Stoneskin spell causes the recipient to turn a uniform grey colour, which is realistic but does get a bit monotonous when your spellcasters are permanently protected by it. Note that this effects NPC mages too. This tweak (which uses Ascension64's "ToB Extender" and so is unavailable on BGEE) removes the colour change from the spell.
Slightly increase the power of Mantle, Improved Mantle, and Absolute Immunity (BG2,BG2EE,BGT)
In the unmodded game, Mantle protects you from +2 or weaker weapons, Improved Mantle from +3 or weaker weapons, and Absolute Immunity from +5 or weaker weapons. In all cases, arguably this is slightly too weak given the prevalence of such weapons in the game. This component increases each protective range by one: Mantle protects against +3 or weaker weapons, Improved Mantle against +4 or weaker weapons, and Absolute Immunity against +6 or weaker weapons (i.e., basically everything.)
The mage AI, in choosing its defensive spells, assumes that this component is installed; if you don't install it, mages may be slightly too willing to use Mantle etc instead of Protection from Magic Weapons, though this shouldn't be too noticeable.
Make spell sequencers, spell triggers, and contingencies learnable by all mages
In the unmodded game, spell sequencers, contingencies etc are in the Evocation school, making them unavailable to Enchanters; this is a serious weakness for the class. This component moves them to be in both the Conjuration and Evocation schools (like Wish), so that all mages may use them. The affected spells are Minor Sequencer, Spell Sequencer, Spell Trigger, Contingency, and Chain Contingency.
Note that if you have Smarter Mages installed, SCS enchanters will use sequencers and the like even if you do not have this component installed.
Add extra spell scrolls for hard-to-find spells (BG2,BG2EE,BGT)
This component adds one extra copy of a few spells into shops in Amn and Trademeet. The spells are chosen to be ones which otherwise tend only to be found late in the game or only given good luck with random loot. The affected spells are Bigby's Clenched Fist, Bigby's Crushing Hand, Control Undead, Improved Mantle, Invisibility 10' Radius, Mordenkainen's Sword, Pierce Shield, Remove Magic, Ruby Ray of Reversal, Spell Shield, Stone to Flesh, and Summon Nishruu. The location of the scrolls is determined randomly at install time.
Give True Sight the ability to prevent magical blindness (BG2,BG2EE,BGT)
This spell modifies True Sight (and the clerical version, True Seeing) so that the caster is immune to Blindness while the spell is effective; existing Blindness effects are also removed.
(The main reason for this change is that Blindness is essentially lethal for enemies (especially spellcasters). With this change, Truesight effectively works like Vocalize, in addition to its other effects.)
Prevent clones made by the Simulacrum and Project Image spells from using their magic items (BG2,BG2EE,BGT)
At present, clones can use magic items belonging to their creator - even one-shot items - which arguably isn't what the spell is intended to do. This component blocks use of useable items, by greying out the quick-item bar for clones. Magic weapons, armour, and protective devices still function normally.