Probably a Smoke, as to the receiver less troops note the first of these lack laser kits on there weapons ( IT's not Miles it's Miles Like) others may not actually be participating as active troops one of the troops who's helmet lacks a smoke plug can be seen talking on a radio, Now in US opfor training there are troops who don't wear receiver kits, they act to keep score and occasionally make chaos they have the "god gun" a master laser that can kill anything on the field from a lowly grunt to a full M1 Tank.
Using this they can kill for IED's or Artillery strikes.
Now Note the "Army" Are Blue tops, Also note the PLA seems to be issuing brown boots. The Boots worn by the Blue army and the REd Marines are both wearing these brown boots which looks like rough out brown leather, Black Sole and camouflaged Nylon looks like either the "Universal pattern" or the Desert pattern
Well, good theory, but I would have expected observers and refs to be issued with clearly different camo so the active participants don't get distracted or waste time trying to 'kill' them and instead focus only on the real opfor.
Also, maybe I wasn't clear before, but if you look, you will see that none of the troopers who lack receivers on their helmets have scopes (and by extension laser tags) on their rifles either. Only a few of them had radios, and those might indeed have been the observers and refs you describe, and some of the guys cutting wires and laying smoke could well have been helping to evolve the battlefield to simulate stuff like artillery and air strikes, but a lot those the guys and a couple of girls if my eyes are not failing me, with no MILES kit look to be active participants. They are taking cover and aiming their weapons.
Firstly, why bother with a blank firing weapon without laser tags? All they will be doing is making noise. Maybe that's the point, but why bother aiming and taking cover if that's the case?