Slightly different from all the other forms of alternate history is the Setting: an alternate history designed not just for the history, but for the end result.
Settings as a format
Of necessity, a Setting is open-ended.
How much information needs to be shared? A Setting serving as a verse for multiple authors requires more information shared to prevent continuity wars. One that's meant for players, on the other hand, probably needs much less pure history; if knowing the specific details of a specific battle in a specific war isn't immediately critical to the setting, you don't need to know about it at all.
- Max Sinister's Chaos Timeline started off as a GURPS setting.
- Tony Jones has several other notable examples.