Skipper is using the same cardinality marks for ORM association like any other ERD systems.
You're controlling owner side optional/required via "Parent connection" property, where you can set 0:1-* or 1:1-*
and inverse side type between one-to-many or one-to-one type via "Association type".
I want to qualify my entities associations into 0,n but don't find such choice in the association options,
"0,n" isn't a complete description of cardinality. You need to configure the owner-side and parent side. Usually something like 0:1-0:n, or 1:1-0:n, 0:1-0:1, etc. First means the parent side, second child (inverse) side.
As I wrote before, the parent side is configured via "Parent connection" where you can choose between 0:1 and 1:1, and child (inverse) side is configured via "Association type" where you can choose one-to-many ( -0:n) or one-to-one ( -0:1).
I hope it's a more clear now.