Kautilya's Arthashastra 2.33.11

2.33.11

tūryadhvajapatākābhir vyūhasaṃjñāḥ prakalpayet /
sthāne yāne praharaṇe sainyānāṃ vinaye rataḥ //E

Being ever mindful of the discipline which his army has to maintain not merely in camping and marching, but in the thick of battle, he shall designate the regiments (vyúha) by the names of trumpets, boards, banners, or flags.