The difference is the directrion the elbow faces. In a standard gutter installation, if you are facing the gutter for example,
an A Elbow is the elbow that will come out off the wall or gutter straight to you.
B elbows would be the ones that would fit the downspout and carry the water either to the left or the right, instead of out (forward) or in (backward).
These do not apply for round elbows.