Each device on a J1939 network has a unique address. The PDU Specific uses device addresses to send parameter groups (PG) to a specific device. Static addresses between zero and 253 are assigned for every device on the network. You can also assign 254, which is a null and 255, which is a global address.
The application sending a PG must claim an ECU address. The application sends an address claiming PG first, and resumes sending other PGs if there is not address conflict. If the source application encounters an address conflict, it can send a PG to the global (255) address to request all devices to declare their addresses. It can then claim one of the unused addresses.