That depends on what the sell system is.
If the sell system is one of your friendly systems (so a Reinforcement action) you
must mine in that system. If you mine elsewhere and sell to a friendly system, you get no merits.
If the sell system is one of another Power (so an Undermining action) you
must again mine in that system. If you mine elsewhere and sell to an unfriendly system, you get no merits.
If the sell system is currently neutral (so an Acquisition action) you
must mine in a Fortified/Stronghold system of your Power in range of the sell system. This is the
only case where mining in one system and selling in another works for Powerplay, and the mining system must be a very specific one in that case.
So, for example in this screenshot:
View attachment 410510
If I do mining in the Ts'in system, and want some merits:
- I can sell those mined goods in Ts'in itself, because that's a friendly system and I'm selling in the same system
- I can sell those mined goods in Chanii, or Rangchan, or HIP 73269 where I currently am (or a few other systems) because those are neutral systems in range of Ts'in, as indicated by the green lines
- I
cannot sell those mined goods in Kipgen Mu or Ru Shui, because they're already a friendly system and only accept locally-mined goods
- I
cannot sell those mined goods in Janijwa, because that's a neutral system which is
out of range of Ts'in (no green line)
- I
cannot sell those mined goods in any system controlled by a different Power
My lines and friendly systems are green because I'm a Kaine supporter - if you were a Grom supporter they'd be orange, if you were an ALD supporter they'd be purple, and so on.
To get the lines to show up, use the "Strategic View" button - the globe icon in the bottom right of the picture.
Though, actually, this one seems decently robust too - it certainly survives relogging and putting unrelated cargo (e.g. Computer Components or Limpets) into and out of your hold, and survives docking at intermediate stations. I wouldn't bet it on surviving anything to do with a FC hold, and we know from mining CGs that it will get cleared if the cargo is jettisoned and recollected, but it's not completely fragile either. I just jumped around the above cluster while testing this out and even the fourth stop after a lot of messing about gave merits properly.
