Choosing the sonar beam setting would be depending on what you want to see under the Xducer...
In your 798 SI....
455kHz is the SI sonar beam....(800kHz is not available in the 798 even tho the HDSI Xducer can support it)...
83kHz is the 60° 2d sonar beam...(wider coverage)
200kHz is the 20° 2d sonar beam....(more detail)
I keep mine on 83/200 most of the time to get the sharpest detail (from the 200)... And still pick up fish in the wider 60° beam (from the 83).....
The only time I turn off the 83kHz is when recording....the 83kHz data does not benefit viewing the recording (in the unit or Humviewer).... But adds a lot of data to the recording which slows down the loading time into Humviewer and takes up extra room on the SD card....
The 200kHz 2d beam cannot be shut off (except by turning the "Sonar" off in the menu)....because the unit gets the depth reading from the 200kHz beam...
Turning off the "Sonar" in the menu makes the unit act as a "GPS/Chartplotting unit" only....
Rickie