Keysight N777 C Series Programming Manual page 15

Introduction to Programming 1
Slot and Channel Numbers
Each module is identified by a slot number and a channel number. For
commands that require you to specify a channel, the slot number is
represented by [n] in a command and the channel number is represented
by [m].
The slot number represents the module's position in the mainframe. The
slot number for N777xC is always 0.
Channel numbers are not used for N777xC.
Keysight N777xC Series Tunable Laser Family Programming Guide 15