Early in the game, Selphie's Limit Break is extremely valuable because it enables you to access spells you probably won't see elsewhere for quite some time. In addition, Selphie has several types of Magic that can't be found anywhere else in game.
When you select Slot, a menu opens in the bottom-left of the screen. You will then see a spell with a number next to it. The spell shown is what will be cast and the number indicates how many times it will be cast if you choose the "Cast" option. If you want, you can choose "Do Over" to receive a different spell and number.
Targeting of the spells is completely random but Selphie will not cast protective/curative spells on enemies or attack magic on allies.