All J1939 packets, except for the request packet, contain eight bytes of data and a standard header which contains an index called Parameter Group Number (PGN), which is embedded in the message's 29-bit identifier. A PGN identifies a message's function and associated data. J1939 attempts to define standard PGNs to encompass a wide range of automotive, agricultural, marine and off-road vehicle purposes. A range of PGNs (00FF0016 through 00FFFF16, inclusive) is reserved for proprietary use. PGNs define the data which is made up of a variable number of Suspect Parameter Number (SPN) elements defined for unique data. For example, there exists a predefined SPN for engine RPM.