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This is a quote from this post by Kromm in the GURPS Forums:

Rulebooks provide new systems, usually either for character abilities (magic, martial arts, superhuman powers, and so on) or designing things (vehicles, weapons, etc.). Their job is to provide expansions of the core rules that can be used in any genre or game world (although they'll suit some genres and settings better than others!). Examples: Martial Arts, Powers.