What are the A B C D E used for?

What you are seeing is the various band bands that are usually referred to as a letter or sometimes a number. Each band contains 40 channels with each channel being on a different frequency. The channels are typically spaced by 10kHz or more. Bands make going to another frequency easy because you only have to remember the channel rather than the entire frequency. In our SR-955HPC for example, there is also a mode called VFO. This mode, allows you to go to any frequency you would like with a zero spacing. It gives you many more options but also can make it things more complicated If you don't know what frequency you need to use. That is where channels can be very convenient.