JL, those are excellent questions.
Abstract words are higher in scope than generalizations or categories. There are only four truly abstract words. They are subject, object, unit and relation. Objects do not have emotional ramifications, subjects can. For example, if a child's toy is broken as an object it is just a broken toy, but if it is as a subject then it is an emotional issue. Units are only used for counting. Relations are combinations of any of these. Relations of objects is engineering. Relations of units is mathematics. Relations of subjects is, supposed to be, philosophy.
Entity and concepts are also abstract words but they, I believe, are special cases of subjects. Entities are tangible subjects, concepts are intangible.
I decided to define subject as "an abstraction for or in a relation" because all subjects are either extrinsic or intrinsic. Extrinsic subjects are used for a relation (to identify a relation), intrinsic subjects are subjects in a relation (they are related together). There simply is no example of a subject one can think of that is neither extrinsic or intrinsic. If a subject has a determinant (a, the, some) then it is intrinsic. If it comes after "of" or "in" then it is extrinsic. Even subjects that do not have these prepositions associated with them do have them implied. There are no exceptions.
Objects similarly only come in extrinsic and intrinsic forms as well. The only difference, like I said before, is that objects have no emotional ramifications.
If you or anyone finds a fault with this I want to know. I'm only trying to get it right.