Message resource refers to a type of message that can be exchanged between systems. Every message is designed with a purpose and the context of the message defines why it is needed and why it has been designed as it has. A message also has a base for its content and that content has an impact or consequence.
The message represents/requests a change also known as the ‘consequence’ that should not be processed more than once; e.g., making a booking for an appointment. It also represents a response to query for current information. Retrospective processing is wrong and/or wasteful. The content of the message is not necessarily intended to be current, and it can be reprocessed, though there may be version issues created by processing old notifications.