Usage distinction between DefinedTerm and CategoryCode

Phil and others, both DefinedTerm and CodeCategory are new schema.org
types. I'm looking for the bright line that distinguishes them in use. I've
been watching the example encodings and I'm not absolutely sure I can
definitively distinguish when one is appropriate to use and not the other.

I think need a bit of narrative explanation since the json-ld examples
alone aren't enough and the definition of CategoryCode ("A category code")
is beyond useless.