Definition

Discrete code. A four-digit SSR code with the last two digits not being "00".

Source: ICAO Doc 4444 PANS-ATM

Description

Discrete SSR codes are usually assigned by air traffic controllers and non-discrete codes are normally used either at pilot's discretion (e.g. in case of an emergency) or when no discrete code has been assigned (e.g. when the aircraft takes off from an aerodrome where no ATS is provided). It should be noted that exceptions exist for both cases. Sometimes controllers assign non-discrete codes. An example of this is the use of conspicuity code 1000 in the European Union for Mode S equipped aircraft who will remain within Mode S airspace until landing (note that the use of this code may have a different meaning in other countries). The opposite is also possible, i.e. pilots sometimes use a discrete code based on e.g. an AIP provision. For example, in the United States, gliders not in contact with an ATC facility should squawk 1202.