In Pro Tools | First, you record audio to audio tracks. Audio tracks can be mono or stereo as appropriate for the recording source.

When recording a mono source, record to a single, mono audio track. A single, mono audio file appears as a clip both in the track and in the Clips List.

When recording a stereo audio source, record to a single, stereo audio track. These files appear as a stereo clip both in the track and in the Clips List.