Er, it's a bit vague I'm afraid but generally it's used to describe a medium paper (as in weight) of the type used in cheap sketchpads these days. In the context it was used you should be able to substitute any white paper with a smooth finish that's not too heavy.
Interestingly given where you're asking this the term might have a military origin, paper of a similar type or weight was used to wrap powder and ball in a neat package before the advent of metal casings.
Einion