ennerishaw, by the nature of your question Im somehow inclined to think that yours is a an MS Flight Flight Simulator question.... sorry, i couldnt think of any other reason why you would ask here, the diference between SEL mode and MANAGED mode (correct me if im wrong, ...I think thats what theyre called) OP CLMB/DES mode etc.
So, from the point of view of a seasoned virtual captain on Wilcopubs great (but buggy) A320 product
Open Climb/Descent is any climb or descent in
Managed Mode (FMGC calculated) that is dictated by virtue of pitch, speed and fuel economy as the primary factor that the AP is maintaining a descent profile/path by adjusting pitch and managing the speed, while the thrust remains at idle(in descent of course), so its not a descent that is in any other
selected mode i.e V/S, CLB SPD APP mode etc. as someone else stated above,. in a perfect world, (as it often is on FS you would climb and and descend to altitude leisurely using open climb. If someone else could correct me or add to this ill appreciate it