"A" bus

Its a buss. You use it to route your audio to and out to the main outs. With it you can use PC if you have Producer and or VSTs to do all sorts of thing with the audio.

I like to rename mine master. I often have many buses to route tracks to than on to the master buss for final processing.

You don't have to use a buss but its a very good idea to do so.

You must have started up a normal template project that is what CW named its buss. You may have others called b and perhaps c. You also have aux 1 and aux 2. These are the same type buss but are often used as auxiliary returns. A buss in Sonar X1 is a buss no matter what its called.

I wouldn't call it fancy routing. Its rather normal for hardware mixers. Most DAWs have some sort of buss structure too.

So you have the master at the far end of the buses. I find it easier to have it at the near end that is to the left in the buss pane.  I use a control surface and I have the first buss as my master which is controlled by a fader on the surface. It also makes it easier to show the master buss when I don't have a lot of room for displaying buses.

There is a lot in the manual about buses that you should read up on.