The minimum requirement for APP should be to match the general functionality of the existing Metaweblog API getCategories method... that is, provide a simple listing of available categories. Management of categorization schemes is potentially a "cool thing to do" but is not a core requirement and does not belong in the spec.
Further, APP should not do anything that attempts to assume, define, or enforce a structure for atom:category terms.
Category Management does not belong in the core.
Do not define a category management mechanism in the APP spec. Where "management" means the creation, modification, and deletion of categories.
Do not define or even discuss atom:category term structure within the APP spec.