Love CAN, beautifully simple. Of course the "arbitration id" needs to be unique to an ECU, but each can send multiple different ones - the purpose of the ID is

Love CAN, beautifully simple. Of course the "arbitration id" needs to be unique to an ECU, but each can send multiple different ones - the purpose of the ID is to always send the one with the lowest number. When designing the system, this ID defines the priority of each message - e.g. brake lights are more important than radio volume up :)