The operation allows to release an assigned SSR code if it is not used for correlation purpose.
Due to the limited number of Mode 3/A SSR codes it is necessary to release assigned codes back into the pool of available codes when possible. Normally this would be handled automatically with the Flight Data Manager or other service monitoring the status of a flight and requesting the release of a code when a flight lands, leaves the Area of Responsibility of an ATSU or is cancelled..

TI Protocol Methods
Processing Consideration