Check scope of uniqueness of identifiers

#interop

In the discussion of JDF-618 we discovered that there still are some identifiers were we did not define the scope within which they are supposed to be unique.

Therefor we should do a check of the specification for identifiers and declare the scope of them. In cases where we can not force the restrictions because of backwards-compatibility we can use a SHOULD instead of a SHALL. This gives directions to new implementors without breaking existing implementations.