Effect Capacity is a hidden measure in Final Fantasy XII to determine how many actions - other than basic attacks - can be executed simultaneously. 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 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 magicks can only be executed exclusively 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 player from performing special actions or magick spells, player is encouraged to opt for weaker spells, especially when fighting the enemies who rely greatly on basic melee attack. 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.