But problem is we dont really know what that flag is. It is just guessed it could be military flag because of triangle shape. In other hand, all flags then were military, IE, they main purpose was to be carried in front of army in war.

Approximately known enough. Truth typology flag is unreliable because there were various kinds of flags in various places, but the gist of it is known. The flag is a signal and identification building in the military structure. They have followed the progress of the battle commanders and fighters at the same time they themselves received notification of their synchronized behavior.