First off, frequency is not everything. It is more complex than that. Still, you can say in general that detectors with frequencies running in the single digits are less reactive to soil and wet salt sand, and also less prone to firing off on tiny trash. They make excellent coin detectors. The 7-8 kHz range is popular for this reason.
As you increase frequency you increase the ability to hit smaller stuff. This is good when you are talking gold. But the detector gets more reactive to the ground and it hits on tiny trash. The detectors get "chatty" for lack of a better term. Salt water is conductive and if you make a detector hot enough it will pick up salt water or wet salt sand. That tends to happen around 15 kHz plus or minus a bit depending on other factors. The AT Pro at 15kHz is about as hot as you can get and still manage wet salt sand but even then it is not that great at it. The way the unit ground balances matters and so even the 19 kHz First Texas machines can compensate for wet salt halfway well with their ground balance range.
But now I am getting too deep into this. Bottom line is the engineer has to decide if he wants the machine to play nice with salt water or not. Once he just says no, who cares, then you can run the frequency very high for very hot performance on tiny stuff. But machines like the 48 kHz Whites GMT or 71 kHz Fisher Gold Bug 2 are so hot that they pick up your hand as they react to the salt content of your blood and sweaty palms! Useless on wet salt sand.
Multi frequency detectors you can simplify by just saying they act like lower frequency detectors. They handle salt water well but are not as hot on tiny stuff as single frequency high kHz detectors.
So say around 8 kHz for machines that tend to be just coin hunters, up to around 15 kHz for do it all good on coins and jewelry/gold plus ok on beaches and above 15 kHz still good on coins and jewelry/gold but no good on wet salt beach sand.
Generalities only but hope it helps.