In the Identifier proposal, classes for identifier types (page 2, point 3) and note types (page 3, point 3) are defined outside the BIBFRAME namespace.  The first example given for identifiers in version 2.0 uses identifier:Isbn as the class, in other words,  However, there is no such thing.  The standard identifiers scheme consists of properties, not classes.   It will be critical to find or create external schemes of classes that can be used for types of identifiers and notes.