What is "Avatar" in the "Animation Controller" used for?

I’m using animation layers for a model of type "Generic".

The "Base" layer contains an "Idle" animation.

The "Shooting Layer" contains a pistol shooting animation.

I want the shooting layer animation to only affect the upper body.
To do that, I need to use an Avatar Mask, I think.

In the avatar mask, I have to select if I want it to be of type "Humanoid" or of type "Generic".

I will not use a "Humanoid" avatar mask because my animations are perfectly tailored for my model, and I don’t need any retargetting.

Instead, I wanted to use the "Transform" avatar mask type.
However, it seems to expect an avatar:

However, I haven’t had the need to create one yet.

Do I really need to create an avatar?
I don’t want any "Humanoid", I want to have full control over my animations and over what’s going on. I have tried "Humanoid" before, and for me, it’s a total loss of control as I can no longer tell what is a bug / mis-use by me and what is automatic retargetting done by Unity, so I want to avoid it by any means.

Why do I have to create an "avatar"? What is it actually?
Unity already has my generic model to work with, so it knows the transform.
Why does it want an "avatar"?

Edit:
The strange thing is: I can create an avatar the usual way -> "Rig -> Create Avatar" -> "From this model".

Then I have an avatar.
In the avatar mask, I need to feed an avatar. I do this, then it shows my transform. Now I can delete the avatar, and it still remembers my transform.

Strange… not sure why Unity can’t simply use the transform.