PM-THG-0010

A Thing MUST be a class. Things MUST be arrangeable in a subclass hierarchy, and that hierarchy MUST be the model's taxonomy.

Non-normative: the description's example is Candidate and Worker as subclasses of Person. Note that this is the same mechanism States use — a State is also a subclass of a Thing — so a Thing's subclass tree contains both genuine subtypes and computed States. That is formally tidy and, per the workbench analysis, not teachable; the resolution recorded there is to keep it normative and never surface the word to a user.