Unknown/unremovable slot
 

Hello,
I generally use Bandit UI to tweak my UI. Everything works fine, however there remains one slot on screen that is a mystery. I marked it on the screenshot:

https://imgur.com/a/jKwpL2v

This slot is not a quickslot, it seems. It always remains empty. The keybinding associated with it is that of 'Command pet', which is used for summoned combat pets. If I unbind this keybinding, it will no longer show under said slot. But a pet never shows in there, nor can I drag one unto it...

Does anyone know which setting / addon ? does this and how to turn it off?

Thanks for your help!

This slot is the companion ultimate slot added by the game.

Thanks so much for the clarification. My companion hasn't unlocked his ultimate, yet.