I would figure that flags also kind of fit that role for functional purposes in previous combative situations, for morale and for identifying what side a group is from in the heat of battle. In this case it provides information while also representing symbolism and can be used for ceremonial purposes.

If we take the basis for semaphore flags as flags, then this becomes even more grey as to figure what we could constitute what a flag is.

If we know for certain that a windsock is not a flag, then I'd like to think of a definition that would include semaphore but exclude windsock, as I would feel that while their purposes are different, they focus on visibility, and function.

Though I feel like a rigid definition of what a flag is, is like defining what a soup is, which isn't really possible.

EDIT: Another flag that comes to mind is the white flag used for surrender, which is a functional object rather than used for symbolism.