I couldn't miss the chance to practice my drawing!
Multi-targeting is a support ability in the Final Fantasy. Usually spells which have this attribute can have their multi-casting effect toggled, while other spells may forcibly effect an entire party.
Most spells can be multi-targeted by having the Materia attached to an All Materia. Mega-All makes nearly all spells and attacks multi-target. This is the only installment where the multi-targeting attribute can be assigned to command abilities.
Note that having Mega-All equipped will immediately overwrite your attack with "Slash-All" even if you have the double cut materia equipped, and you will only attack all enemies once. Mega-All is also incompatible with Quadra Magic, the screen will show that you are targetting all enemies/allies but only one random enemy/ally will be selected and all 4 castings of the spell will affect only that one target.