Programme Identification (PI) is a service provided by radio stations transmitting Radio Data System (RDS) data as part of the FM radio broadcast. It is 4-digit hexadecimal number.
The PI code allows the radio to identify the station across different broadcasts / multiplexes. For example BBC Radio 1 has PI C201. It is also used in Digital Audio Broadcasting as Station ID.
In allows listeners to stay tuned to a network whilst travelling across the service area of multiple transmitters. It also allows listeners to a particular network to benefit from cross-network travel news services, traffic flashes and for the flagging of travel-related programming.
The PI Code is also used by the RadioDNS standard.