The heading or HDG is a technical term that indicates the direction of the heading, which in the absence of a lane coincides with the COG. If you know both the COG and the heading, you have the possibility to calculate the drift.
The heading is expressed in degrees and can be true or magnetic depending on the device that reveals it. It is normally detected by a compass, gyro or electronic compass. The GPS is not able to detect the heading but only the COG.
Radars to operate in full functionality need this data which can be received via NMEA by one of the devices capable of calculating it.