The Channel map
In this dialogue you can see all the video tracks I have on my timeline in the column and my output tracks along the top. This shows the default number of audio tracks in a project. Just click on the word "mono" or "stereo" to flip between the two settings.
Stereo means that any clips on that timeline track will go out to the two channels the word stereo spans and how loud it is on one track or the other is defined by the panning settings on the audio.
Mono means whatever goes on that track will be output onto that audio channel in the final file and any PAN settings are ignored. So in this example everything on the VA and A1 track are going out to the 1st audio channel, everything on 2A is going out to the second, and so on.
If I only had two audio tracks (which is the default for stereo output) then I would only have two channels along the top. If I was trying to get a surround sound file I would have 6 channels along the top.
Changing the channel map in the project settings changes it for every new timeline in the project. If you want to change existing timelines or have different channel maps per timeline right you can change the channel map in sequence settings in the same way.