Flags are used when creating Allow and Disallow signs. The flags go in line 3, directly under the player's name. Leaving line 3 blank is the same as using the * flag.
Each flag is represented by a single letter.
Below is a table showing which flags do what.
|O||Open Wooden Doors|
|S||Use Special Blocks (furnace, dispenser, brewing stand)|
|U||Use (buttons etc)|