Refer to figure.
In practice the ATC uses both the primary and secondary surveillance radar (SSR) systems together. Primary radar is a very useful equipment in determining the aircraft bearing and range, but it has limitations - for example if the target is too small, targets can not be directly identified as a specific aircraft (no transponder code) or the radar signals suffer attenuation due to adverse weather such as storms. For these reasons the ATC uses both the primary and secondary radars - the SSR supplements the primary radar.

When used in conjunction, the primary radar is more accurate in determining the bearing and range of the targets than the SSR, but the SSR provides multitude of additional information - the ATC controllers are therefore able to see the squawk code (transponder code), flight level (provided by the transponder in mode C), groundspeed calculated by the SSR ground facility based on the change of bearing and distance of the aircraft and a callsign (provided by the transponder in mode S or assigned by the ground facility to a specific squawk code).

Note: This is examining LO 062.03.04.03.02: State which information can be presented on the ATC display system: pressure altitude; flight level; flight number or aircraft registration number; GS. The actual capability of Radar displays may vary between countries/military/civilian use.

Important Note:This has been seen recently (Dec '25) in the UK as a 3 drop down question where each option had to be entered separately.