MultiDetect is a new feature Teledyne RESON have implemented which allows the sonar to export multiple returns per beam, thereby giving you soundings on objects in the water column.
If you are surveying over a fish ball, you get the strongest return from the fish, which the sonar traditionally designates as the "bottom". And then you are left with a hole on the seafloor. But, with Multidetect, you will also get the 2nd return that made it through the fish ball as well.
Below is a perfect example of one advantage of MultiDetect from a SeaBat T20-P demo we did with last week in Tampa. Surveying over a large school of fish, you can see the fish in the water column and all the corresponding holes in the bathymetry data on the seafloor underneath.
At the red arrow is when we turned on MultiDetect. At that point, you still get the fish in the water column, but you also fill in the "holes" on the seafloor that were shadowed by the dense school of fish.