In the Audio Editor, you can select individual channels, both channels of a stereo file, or channel clusters of a multichannel audio file. This allows you to apply an operation to one channel only, to both channels of a stereo file, or to channel clusters of a multichannel file.
Which channel is selected when you click in the Waveform display of the Audio Editor depends on where you click.
The edit cursor shows which channel is selected. The mouse pointer indicates which channel is affected.