A repository may automatically assign a mixin type to a node upon creation. For example if, as a matter of configuration, all nt:file nodes in a repository are to be versionable, then the repository may automatically assign the mixin type mix:versionable to each such node as it is created.
Similarly, a repository may automatically strip incoming deserialized nodes of any mixin node types that the repository does not support (see 7.3.3 Respecting Property Semantics).