I assume this is an Airbus question...
If the vertical mode of the FMA reads ALT, then any selection must be followed by pulling (selected) or pushing (managed i.e. FMGC).
If you are in a climb or descent (OP CLB/CLB, OP DES/DES) then selecting a new level will be the new target to level off at.
Managed CLB will climb at managed speed (FMGC) at climb thrust (THR CLB) observing any constraints... V/S is a different mode, so setting a V/S will have no effect until you select it (pull). Now you are climbing (or descending) in V/S mode...
On approach, (GS* or GS) selecting a different target altitude will have no effect until you set TOGA (assuming you have set a higher altitude...) then it will be the new target altitude to level off at IF you go-around or discontinue the approach.
Regardless of vertical mode (unless protections have kicked in...) PUSH TO LEVEL OFF will over-ride the previous mode and V/S will be zero (thrust engages in SPEED).