Use
From Final Fantasy Wiki
Use is an ability appearing in Final Fantasy X. Initially available to Rikku and later to the entire party, the ability enables the character to Use any of the special items present in the game not accessible via the Item command. These items are necessary for Rikku's Mix Overdrive and on their own can inflict damage, or heal and buff the party.
Most of the items usable in battle require the ability Use in order to be activated.
Some items that many players will often Use (pun not intended) are: Al Bhed Potion, Twin Stars/Three Stars, the various "curtain Items", and (if available) Dark Matter.
Most Use Items are offensive or supportive in nature, with Al Bhed Potion, Healing Water, and Tetra Elemental being the only Use Items that actually recover HP (Healing Spring doesn't count, as since its effect is Regen, it's supportive).
