How is an aircraft reported at other than the assigned altitude indicated on the flight strip?

An aircraft reported at an altitude other than the assigned altitude is indicated on the flight strip by circling the altitude in black. This visual cue helps air traffic controllers quickly identify discrepancies between the aircraft's reported altitude and its assigned altitude, which is crucial for maintaining safe separation and ensuring effective communication regarding altitude changes.

The use of a black circle draws attention to the deviation, allowing controllers to promptly assess the situation and take necessary actions to resolve any issues related to altitude deviations. This standard practice reinforces situational awareness for the controller while efficiently conveying critical information at a glance.