The purpose of the Late Roman Draco standard.

[ValentinianVictrix]

My belief is that the vexillum (Labarum) standard was to indicate the unit it belonged to, so that each unit it was attached to on the battle field could be seen and then directed by the commander of that portion of the army it was in. It also would give commanders an indication of whether the unit was advancing, retreating etc. I'm still unsure of the purpose of the Draco standard, the interpretation that it was used to indicate the position of the unit commander is not unreasonable.

It's also not unreasonable to suggest that most, if not all, battlefield commands were given by audible signals, as indicated in a number of passages in Ammianus.