The Equip Previous item ability will equip the previous available Item Set within the specified category. If the current Item Set category is at index 3 and the Equip Previous ability starts it’ll then try to equip the Item Set at category index 2. If this index cannot be equipped (such as if the inventory doesn’t contain the item) then it’ll try index 1. The ability will try every index in linear order until it finds a valid Item Set or reaches the starting index again. The Equip Previous ability doesn’t actually do the equip or unequip – it instead relies on the Equip Unequip ability to do the actual work.