The purpose of the Late Roman Draco standard.

[ValentinianVictrix]

I cannot agree with all of the above. The Labarum standard as used by Constantine the Great was a specific standard associated with him only. It was stored after his death in Constantinople. There is pictorial evidence of vexillum standards bareing the Chi-Rho symbol, this symbol was not limited to any standards donated by an emperor. It is probably linked to the claim that Constantine dreamt that God commanded him to have his army scribe the Chi-Rho symbol on their shields etc and he would be victorious. We have monumental evidence and line drawings of destroyed monuments that show troops upto at least the reigns of Arcadius and Honorius having shields with only the Chi-Rho on them, clearly they were not donated by the Emperor.

Yes, "The Labarum" was a singular talismanic standard used by Constantine the Great, and recorded as late as the 10th century as being housed in the imperial treasury in Constantinople. However, as I have said earlier, Eusebius stated that Constantine gave multiple labara (apparently visually distinct from The Labarum) to units in his army. Indeed their multiple status is confirmed on the coin of Vetranio, as a soldier is portrayed holding two such cho-rho monogrammed standards. A silver plate showing Constantius II has him accompanied by a guardsman bearing a chi-rho decorated shield, but shields are not standards so I don't see their relevance to a bald statement by a contemporary writer that an emperor donated labara to units of his army.

I think modern authors counsel some caution when reading early Christian writers, they had an agenda to ensure that their faith was promoted at every opportunity. We cannot be too sure if what Eusbius states is bald fact or was it Christian propaganda?