What does the "Dismiss familiar" button do when it is a pet?

What happens when you click on the "Dismiss familiar" button when it's not a summon but a pet? By that I mean the rightmost button in:

Does the pet return to inventory? Does it go away forever and I lose the pet?

I'm not willing to press the button to test the outcome after waiting so long for it to grow into an adult and I'm hoping someone else already has and can tell me what that button does.

When you press "dismiss familiar" on a pet, you get the following prompt:

There's no real reason to press "dismiss familiar" on a pet, since you can just left click your pet to pick them up.