Does it not just mean you're being moved to another FARP or base to supply frontline forces? I think I've seen this before.
That would be a Transfer.
Ah, I knew there something like that. It's actually been so long since I've played the game properly. I've not started a campaign for well over a year. Most of the time I'm just testing things.
So to get back to the OP, what's causing a non-transport helo to be assigned a Supply mission?