@acetrainereddie
There are actually more flags available, check
this source to see where Fire Red flags are used in Vanilla Fire Red.
@Pokechu
Although you're a little right about flag 0x200 - 0x2FF, what you say is not all true and based on pretty old research. The statement in karatekid552's thread to only use flag 0x200 to 0x2FF was just to keep things from getting complicated. Saying that ''the game will slowly break when using other flags'' and that ''those are the only available in the game'' is plainly false.
Check the documentation on pokefirered I sent to @acetrainereddie to see a (nearly) completed list of all flags available in Fire Red or check Karatedkid522's full findings on where which flags are used.