Allen-Bradley 1779-KFMR User Manual page 107

We show both the application-layer fields and the Data Highway fields to
indicate where the application-layer fields fit into the message packet.
Command Format:
Success Reply Format:
Error Reply Format:
CMD (Command) and FNC (Function) — The low nibble of the
CMD byte is an application-layer field. For those message formats which
include an FNC field, the low nibble of the CMD byte together with the
one-byte FNC field define what action the command executor at the
destination node will perform. For those message formats which do not
include an FNC field, the CMD field alone defines what action the
command executor at the destination node will perform. FNC fields are
only in commands. Bits 0 through 3 (the low nibble) of the CMD field
must be the same in the reply message as they are in the corresponding
command message. The value these bits represent is a command code or
a command executor selector. The application program must always copy
this field from the command message to the reply message.
Chapter 8