A total of 64 message types were allowed, although a number of these were deliberately left unused for future expansion, or formats that were rarely used and later abandoned. The original standard includes six message formats, 1 for correction data, 2 for updating previous corrections, 3 that provided the location of the measuring station, 6 as a null message to fill unused slots, 16 which included an arbitrary 90 ASCII characters for sending test messages, and 59, for proprietary messages used by the equipment vendors.
