Whether the message is a broadcast.
The destination address for an addressed (PDU1) message, or null for a
broadcast (PDU2) one.
The PDU format byte of the parameter group.
The parameter group number, which names what the message carries.
The message priority, 0 (highest) to 7.
The source address: the node that sent the message.
The fields J1939 packs into an extended CAN identifier.