The application layer header function code indicates the purpose, or requested operation,
of the message. While DNP3 allows multiple data types in a single message, it only allows a
single requested operation on the data types within the message. Example function codes (as
shown in fig. 4) include: Confirm (for application layer confirmations), read and write, select and operate (for select-before-operate, or SBO, controls), direct operate (for operation of controls without SBO), freeze and clear (for counters), restart (both cold and warm), enable and disable unsolicited messages, and assign class. The application layer header function code applies to all object headers, and therefore all data within the message fragment.