Content has two aspects. The first is the mechanical completeness and accuracy necessary to a set of rules that is designed to allow simulation of life. The second aspect is the original or literary aspect. It is the implied world and setting that goes with the rules independent of genre. The last part of a rule book is genre.
Rules can cover an amazing variety of imaginary worlds.
Generally, these variations are thought of as genres and sub-genres. Call of Cthulhu, originally set in the early twentieth century, recreates H. P. Lovecraft's novels and short stories accurately and completely. Stormbringer does the same for the works of Michael Moorcock. Rather than explain genre and sub-genre points at length, I would refer anyone to the Fantasy Hero rules. Those rules contain an exhaustive section which explains the elements of the most common genres and how to use them in a game setting.