The Crimson Binome
Hero
The PHB presents everything with a bunch of default assumptions, and then the DMG turns around and tells you how to change those assumptions in controlled ways. The gods are assumed to be real and to show up from time to time, but some settings might not have them be so active (Eberron), and some settings might have them completely cut off from the world (Dark Sun).I was wrong, not DMG but PHB, First paragraph of the Classes chapter. Not really put in as an optional statement but rather as a description of what player characters are.
Likewise, the PCs might be fairly rare and powerful individuals, or there might be a lot of others in the world who are just as strong as they are (if not stronger). Your typical bartender might be a level 0 chump, or she might be a level 9 ex-hero with a merciful greatsword. It depends on the setting.