[CONNECT C++] Delete Item Type Library: reasons for failure

I'm writing code to delete an Item Type library from a DGN file. ItemTypeLibrary.Delete() returns a SchemaDeleteStatus value.  While some values are clear, what do the following mean?

1. SCHEMADELETE_ProviderNotFound: what is the provider if not the active DGN file — a DGNLib?
2. SCHEMADELETE_ProviderDoesNotSupportDelete: presumably because it's attached read-only?
3. SCHEMADELETE_SchemaHasStoredInstances: instances that are attached to DGN elements, or something else?
4. SCHEMADELETE_SchemaIsReferenced: referenced by what — another schema or something else?