Don't think it's that complex. If your refering to the /6000B constrain on an LHR SID the aircraft climbs to whatever is set on the mcp or the constraint whichever is lower, until you press the alt sel knob to erase the constraint.
If you then turned round and flew a descent into LHR or continued to JFK the constraint would apply the other way round. It would adjust the top of descent point to acheive /6000B by that waypoint.
I'm not sure it has anything to do with acheiving cruise alt/lvl.
Hope that helps.