As already posted,
If you place the unit at "A", you will generally hear noise, at "B", your squelch is at the max, meaning you generally won't hear any noise. Also, at the "B" setting, low power or weak signals may not open the squelch and you could miss a signal.
Way I do my radio's squelch is to tune in a frequency or channel (that's not busy or signals coming in) and turn the squelch dial to where I can hear the noise floor, and then turn it back to the "B" a little at a time to where the noise ceases.
Hope what's been posted will help you decide where to set the squelch for your listening enjoyment. Scan away...