Playtime, I have never used your particular sounder, but i can tell you a few thigs about low vs high frequency in general.
Firstly, your high frequency mode give you your best detail. It works fine till you start getting into deeper water, where it suffers more losses and will eventually be useless. This depth will alter with different sounders and different power output capacities. No idea on yours, but this would be over 100m at a guess..
Low frequency penetrates the substrate (in this case water) much better, (same as HF radio working over greater distances than VHF) and so is handy for picking up bottom once over the shelf. If you never go that deep, you will probably never have to use the low frequency setting.
If in shallow water using LF, you may find that because the signal travels further easier, it picks up a lot more 'false' echos (signal rebounding from bottom to water surface, to bottom to surface and so on), with your transducer-receiver picking up a bit of the echo each time it rebounds.
i am sure someone on here will be able to provide a better explanation, but i hop this helps you understand the difference (LF vs HF) a bit more.
If all else fails, consult the manual
Damo