Could be wrong...I recall a reg somewhere from my old L1011 days that the last 1000ft of altitude capture in a climb to be required to be completed at a rate of at least 500ft/min.
i.e. ATC did not want you creeping up at a rate of 50 ft per min on fuel burn as it mucked up their planning!
Also the last 1000ft in a descent to altitude or flight level be reduced to a rate of 500ft/min....I believe(probably wrongly) the B767 auto pilot/computer logic was designed to do this in Flight level change mode.