Effect Capacity is a hidden measure in Final Fantasy XII to determine how many actions—other than basic attacks—can be executed simultaneously. In the PlayStation 2 versions, if the total effect index exceeds the maximum capacity of 8, the action charged last can only be executed when the earliest action ends. This is likely due to the hardware limitation of the PlayStation 2 being unable to handle too much graphical processing.
The effect index for most tier 1 magick spells is 1, tier 2 (-ra) 2, tier 3 (-ga) 4 and the strongest spells 8, meaning the most advanced magick can only be executed when no other special actions are taking place. Some animations, such as Darkga and Graviga, may look simple, but have higher effect indices than other -ga magick spells.
Because this system hinders players from performing special actions or magick spells, the player is encouraged to opt for weaker spells, especially when fighting enemies who rely greatly on basic melee attacks. For example, casting Cura and Curaga under Faith status instead of Curaja is more efficient because they cost 3 effect capacity units and consume less MP (32+28=60) altogether, compared to 4 effect index and 68 MP of the latter. Knowing the enemy's pattern allows skillful players to stall the enemy by saturating the effect capacity, regardless of the enemy-exclusive skills with zero charge time.
Due to the greater technical capacity of the PlayStation 4, the Effect Capacity measure is greatly expanded in The Zodiac Age version, allowing nearly all spells to be used the moment the ATB gauge has finished charging.
Effect index 1
|White Magick||Cure, Curaga, Raise, Esuna, Blindna, Vox, Poisona, Stona, Cleanse, Dispel, Regen|
|Black Magick||Fire, Blizzard, Thunder, Water|
|Time Magick||Haste, Slow, Stop, Break, Countdown|
|Green Magick||Protect, Shell, Bravery, Faith, Blind, Silence, Sleep, Poison, Oil|
|Arcane Magick||Dark, Reverse, Berserk, Confuse, Decoy, Vanish|
|Technicks||All, excluding Shades of Black and Telekinesis|
|Items||Potion, Ether, Remedy, Phoenix Down, Gold Needle, Echo Herbs, Antidote, Alarm Clock, Handkerchief, Chronos Tear, Smelling Salts, Vaccine, Bacchus's Wine|
Effect index 2
|White Magick||Cura, Arise, Dispelga|
|Black Magick||Aero, Fira, Blizzara, Thundara, Bio|
|Time Magick||Slowga, Immobilize, Disable, Reflect, Bleed|
|Green Magick||Protectga, Shellga|
|Arcane Magick||Death, Bubble, Drain, Syphon|
|Items||Hi-Potion, Hi Ether, Knot of Rust, Red Fang, Blue Fang, White Fang|
Effect index 4
|White Magick||Curaja, Renew, Esunaga|
|Black Magick||Aeroga, Firaga, Blizzaga, Thundaga, Shock|
|Time Magick||Hastega, Reflectga, Float, Balance|
|Green Magick||Blindga, Silencega, Sleepga, Toxify|
|Arcane Magick||Darkra, Gravity, Vanishga|
|Items||X-Potion, Elixir, Megalixir|
Effect index 8
|Black Magick||Scourge, Flare, Ardor, Scathe|
|Arcane Magick||Darkga, Graviga|
|Items||Eksir Berries, Dark Matter|
|Misc.||Summon, Quickening, Boss and Esper's ultimate attack|
- Actions that ignore these mechanics include any basic regular attack regardless of element, Telekinesis, enemy exclusive Technicks, such as Rush, Lunge, Ram, Rake, Kick, Gnaw and Poke.
- In the Zodiac versions, some spells have been re-categorized into different magick classes, but the effect index remains the same.
- Magickal Motes and the magicks drawn by Shades of Black have the same effect index as the corresponding spell.