'To scrim' basically means your adding real or fake foliage to something to aid in camoflage. The term scrim just stuck to the nets attached to helmets that you use to attach the foliage.
That was my helmet at the beginning of SQ, with the scrim net attached. Basically the net keeps tight with an elastic band and keeps attached by tucking the plastic tabs into the helmet liner. Anybody who's seen the Gallet and attached the regular cover will know what I mean