message data structure that has been associated with a specific meaning and which, when properly sent in a data packet, an instance of the structure can convey information between systems

NOTE A data structure might, for example, be specified to include a list of speeds from detector stations. This one data structure could be used to specify the content of several messages (e.g. the list of current speeds being detected, the list of stored speeds that will trigger a congestion warning if current values fall below the indicated level, or a request for a list of locations where the current speed is less than the indicated speed). An instance of the message would then contain actual values.