Each major component of an entry has a corresponding type of category.
Well, duh.
In principle, all lemmas should be in an etymology category of some kind, even if it is only on the basis of their etymology being unknown. These may be supplied either by an etymology template, or at the end of the language section.
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These may extend beyond the limited set of terms permitted in POS headers, to inclue "prepositional phrases," "idioms," etc.
These categories may be based either on pragmatic characteristics (vulgar, pejorative), or on the field or genre in which they are used (sci-fi novels, patents, biochemical literature). They are typically assigned at the beginning of the relevant sense, either by {{context}} or a related template.
These are based on the actual meaning(s) defined in the entry. Categories based on semantic affiliation are bound to part of speech; any entry which is properly placed in Category:Animals is also a noun. Where possible, semantic categories should be children of a fundamental category
These are generally assigned by a cleanup template.