Augments
taken from Wikipedia

The system allows for certain character-only abilities to be transferred to other characters who did not have them in the original and previous releases of Final Fantasy IV. Up to three abilities can come from temporary party members. When leaving the party, temporary characters will yield abilities of their own, the number of which is dependent on how many abilities they were given. There are also other abilities; some scattered around the world, and some that become available after certain story events.

This new system entails another new feature: command menu customization. All commands in a character's battle menu except the Items command can be replaced with augments. This includes individual abilities that are ordinarily contained in a group (e.g. Curaga can be added directly to Rosa's command list, rather than only being accessible through the White Magic sub-list).

The Augment System was devised to replace the system in Final Fantasy IV Advance where the characters that were temporary in the original version became playable again at a certain point. The developers felt that this system changed the game too much.