I haven’t been able to drum up a concrete definition of flags, but the gist is that flags tell you what a player is interested in encountering during a game. They are often (always?) tied to characters.
In this thread over on SG, a number of types of flags are suggested. I liked these three.
- Fixed: This aspect of the character is not intended to change. The player isn’t interested in a conflict about this character aspect.
- Gradual: This aspect can change, but is not intended to change right away. It can be introduced, but the player prefers that it is not forced.
- Sudden: This aspect can change, and is intended to be a focus for conflict at some point. The player doesn’t mind if this is brought into the story rapidly.
I’d suggest a fourth.
- Urgent: This aspect is a flashpoint for change, and is intended to be an immediate focus for conflict. The player wants this brought into the story immediately or as soon as possible.
This bleeds over into the realm of technology when I suggest that a method of character creation that encouraged players to categorize their background information into these bins would be very informative for a GM, and hopefully help them introduce the kinds of conflicts that the players are looking for.