Unknown/unremovable slot

[Hotzelvit]

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!

[Anceane]

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

[Hotzelvit]

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