As far as I can tell, they are all carrying ore and moving as one fleet (yes at the same time). For example I set 20 cargo ships to move ore from an outpost that had accumulated CO to a colony in the same system and the fleet appeared to me to be moving 50-60k CO per trip, meaning all 20 were loaded up. When I cancelled that harvesting route to send them elsewhere, I only had to cancel the fleet movement and then I could send the ships elsewhere (still as one fleet, without the need for any fleet screen management).

Works great and I wish I had realized sooner. It's way better than having to send one ship at a time when moving large quantities, though I still use individual actions when needing 1-3 harvesters to move ore between planets on a permanant basis.