dual beam is a unit that can send out both 83 and 200 signals, there is a option in the menu to select one or the other, can't use both at the same time and commonly rerferred to as 2d. Down imaging is a newer type and will give you picture like images on the HU screen with much more detail than 2d will. You will think you're looking at a photograph of the lake bottom with down imaging, the detail is very good. Look in the electronics forum for some pics to see the differences between the two.