There are other differences in equipment and positions allowed for Prone Only, but the biggest difference is between targets.
Conventional 50 yard NRA is shot at 50 yards on a target with a 10 ring of 1.70 MOA. It is shot prone, with sling support only.
Metric 50 meter Prone is shot on at 54.8 yards on a target with a 10 ring of only 0.715 MOA. It is also shot prone, with sling support only. HUGE difference.
I don't pay a lot of attention to F-Class. I think they shoot the same targets, but off of bipods, bags, one-piece rests constructed within the rules, etc. I suppose it could be good for hammering out a shooter's ability to call wind.