Level change:

Reported flight level requested by ATC:

(After some time)

Level change using conditional clearance:

Once having been given an instruction to climb or descend, a further overriding instruction may be given to a pilot:

Usually at first contact in cruise, ATC can request pilot to maintain current altitude:

Occasionally, for traffic reasons, a higher than normal rate of descent (or climb) may be required in order to free the higher flight level left:

As a pilot if you are unable to follow the expedite clearance you shall report that to ATC:

Once having been given an instruction to climb or descend, a further overriding instruction may be given to a pilot: