How to define a mechanical layer

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I do not know if I am really missing something but why there are no default mechanical layer option in KiCad?
The only way to make a mechanical layer, it seems I have to Add a user-defined layer, change its name to Mechanical and then give it a layer type. Auxilary, off-board front or off-board back.

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Specifically, what I want to do now: the fab house asks me to place the depth routing area (partial milling) of my PCB in a mechanical layer to define it.

So 2 questions:

  1. Is this the proper way to create a mechanical layer? If yes, which of the three type options should I pick for the mechanical layer?
  2. Why there is no mechanical layer by default? KiCad has so many extra layers like User.Eco1/2, User.Comments, User.Drawings and User.1 to User.45 to add. But I would have though mechanical layer is more important than all those and should come first.
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IMO, yes . . . is your mechanical detail applicable to both sides of the board, e.g. the board profile, if it is then use Auxiliary

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if it is side specific (a routed blind profile probably would be) then choose if it’s the Front or Back of the board that you want to specify info for.

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At a guess . . . mechanical layers are most often used for things like board profile, KiCad has the Edge.Cuts layer for that so a dedicated Mechanical layer is not often needed.

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