Raymarine GPM400 Reference Manual page 94

• Higher frequencies (200 Khz) scan a narrow area but produce
more detail, especially at high boat speed. They are most useful
in shallower waters (up to 1000 feet).
Various frequencies may be available on your system, depending
on the transducers fitted to your vessel.
Viewing single or dual frequencies
If the preset you are using includes two frequencies, you can use
one fishfinder window to view both frequencies or just one. You can
then send the second frequency to a fishfinder window on a second
monitor, enabling you to view both frequencies at full screen.
G-Series Reference Manual 94