When passing traffic information, the Mil only refer to traffic being "At" a level, altitude or height, when that traffic has been co-ordinated or under the control of the same controller and the Mode C has been verified. With regards to the original post, by inputting the regional QNH into the radar every hour, Mode Cs below the Tansition Altitude are calculated by the computer to the same QNH and displayed as an altitude, ie A30. When aircraft climb/descend through the TA, the Mode C will adjust on the display to a 1013 conversion. Therefore, during periods of high/low pressure, an aircraft displaying FL060 can in fact be at the same level/altitude as an aircraft at 5500 ft on the regional QNH.