Most multiple frequency metal detectors output between 3 and 5 frequencies simultaneously. Some output more. The main advantage of this technology is if a target is missed by one frequency, another might be able to pick it up. For example, the lower frequency of 5 kHz may not detect a small piece of gold 6” down.  But the 20 kHz and 40 kHz frequencies have a chance of picking up this piece of gold as the detector utilizes its multiple frequencies. Multiple frequency metal detectors are much better in saltwater areas than single frequency detectors. As I stated, this can come at a loss of performance as there is much more data for the machine to interpret.