In each hotbar slot, you can choose a type of usable item, a specific pet, or a specific equipment/vanity loadout.
If a pet is in a slot, using the slot summons that pet. If you can't currently summon that pet it is greyed out (already summoned or current pet ability on cooldown).
If a loadout is in a slot, using the slot applies the loadout. If the loadout items are already equipped it is greyed out.
Bookmarks