But wouldn't Approach know your assigned alt, any speed restrictions, waypoint time requirements etc? Doesn't all this stuff get forwarded down the line?
............And the aircraft has (most of the time) been 'identified' on radar!
Yes, we put assigned level, speeds, etc in the track label. However there is no record of a previously assigned level.
For a controller to know you're maintaining a level solely by looking at the mode C readout it needs to be referenced to an assigned level. Approach doesn't know you were assigned F200 so unless you tell them the mode C can't officially be used to say so. Pedantic perhaps but them's the rules: within +/- 200ft of the assigned level.