Afaik, its more like a second "set" of way points. If you have a flight plan with say, 3 way points and you make another 3 mark points, when you start cycle them, it will go WYP 1, WYP 2, WYP3, MRK 1, MRK 2, MRK 3. Once selected, they can be edited just the same like way points.