2d broadband sonar is the old format sonar that sends the sonar ping/signal down in a cone shape.
Depending on the degree of the signal, you get a wider shaped cone and therefore greater coverage area. A 20deg cone angle signal will show a circle on the bottom that is 1/3 as wide as the depth of water ... so in 21ft of water the cone will be 7ft wide on the bottom. A 60deg cone angle will show the same width of the circle as the depth ... so at 21ft deep, the cone circle on the bottom will be 21ft wide. But, you have to understand the shape of a cone is /\ so it's not going to cover/show as much in the upper portion ^ as it does in the bottom portion.
Also of note : a 2d screen image is going to look like a "finger painting" compared to a DI screen image, which will look much more like a photographic image.

DI signals are not circular or cone shaped, like a 2d signal ... they are oblong fan shaped signals that are only a few degrees thick (~6deg front to back) but extend out at greater angles than a 2d cone. The 800kHz signal is 45degs to each side, while the 455kHz signal is 75degs to each side. They're somewhat like SI sonar signals, except they do not extend to 90degs on each side, like the SI signal does.

This chart will give you an idea of shape & coverage area/angle of the different sonar types :

newDIbeams.png Photo by kosmo012 | Photobucket

... cp