I'd imagine (no sources here, just trying to use some common sense) it pertains to "on duty" (as far as any reserves would be concerned) when you're actually being paid to be on duty. Operating in the official capacity of the military. On a convoy, stopping for gas/chow at any gas station, restaurant while armed you'd be fine carrying a duty/TO/issue weapon. Drill over, changed into civvies, or still in uniform, carrying your personal piece (non-issue), picking up your kid at school (not official military business or in an official military capacity), you're not good.
ive seen this topic kicked around several times. No one I know would test this theory.
I do agree it does hinge a lot in the definition of Duty. So stepping away from a drilling reservist, how would it apply to an active duty person, someone paid 24/7 for their service. More so, how would it apply if their duty day consisted of being out and about in the community, say as a recruiter?