Canada: When used in conjunction with an altitude clearance, means that ATC has offered the pilot the option of commencing the altitude change whenever the pilot wishes. When the change is initiated, the pilot should advise ATC. Pilots may temporarily level off at any intermediate altitude; however, once an altitude has been vacated, the pilot may not return to that altitude because ATC may have reassigned it to another aircraft. Pilots are expected to advise ATC of any temporary level-off at any intermediate altitude. Compliance with any assigned or published altitude crossing restriction is mandatory unless specifically cancelled by ATC.