For alliances A flag is the symbol of your alliance. A bad flag will probably not hurt a strong alliance but an ugly flag can sink a weak one. Therefore, it is important to make sure that your alliance's flag is presentable. In terms of what to put on the flag, the simpler the better. Your flag should incorporate the color of your alliance's home sphere and should contain symbolism that evokes the name of your alliance.
For nations A symbol of your nation.It should be simple and have some relation with the principles of your nation or you.