For your fighters, you can set up to two Special Arts. You’ll need to set them up here before you can use them in battle. Even your AI-controlled allies use Special Arts without setting them up in the L/R buttons first.
You can also set up two offensive spells to the L/R buttons as shortcuts for your spellcasters. This will allow you to cast them easily without pausing the battle and browsing their Symbology menu.
For spellcasters, you have the option to toggle which spells they'll use when controlled by AI. It is strongly suggested to turn off lesser spells once you have acquired stronger ones as you level up. For example, you can turn off Cure Light and Cure All heals once your healers have access to Faerie Healing and Faerie Light, which can restore more HP. For support or buff spells, consider turning some of them off since your spellcasters will prioritize casting them.