For testing the loss on single mode fiber, use the CW selection. CW stands for "Continues Wave" which is used for optical loss or output power measurements. MOD stands for "Modulated" which is used for identifying fiber cables with a fiber identifier. The MOD setting modulates the source output at 2 kHz. To change the switch position, pull out on the switch arm; then move the switch.
Looking for the manual? Click here.