Standard layers used for "non- essential" things when creating parts?

[reagle]

Say I am creating a new part and need to define things like mechanical outline, courtyard, assembly layers etc. Depending on the library I look at, those end up on different mechanical layers. One library  would use layer 20 for courtyard, while Altium's own IPC wizard would use layer 15. Is there a standard somewhere defining what they should be or is this all up the user/librarian?
 

[free_electron]

1 and 15 are top and bottom 3d model.

But it really doesnt matter.