From Fortunate Blades

Backgrounds shape where a Character comes from and gives them a Talent of some kind.

When you create a character, you pick two of them, and combine them together to make an idea for the backstory of that character.

The Player of that Character then gets to fill in all the rest of the details of that backstory, as they are now the 'main authority' about it.

For example, if you are playing a Character with a Military Background, you not only write down some Facts about it, if you wish to you can go into more detail, and those details become 'cannon' and part of the Story.

If you don't wish to do so, then GM can can make up more of those details, or better yet, make them up together by playing the game.

But a Player always has 'veto' over details created by others about their Character background.