Dumb question regarding input.

You can use either Left or Right to record Mono Audio tracks. Use Stereo to record a Stereo Track. That is, one audio track that will contain separate left and right sides. If you're recording something like a vocal, direct guitar/bass, or anything recorded with one microphone... all you need is a Mono Track. Doesn't make much difference if you use Left or Right because that has nothing to do with its output panning.

And, of course, it depends on how you're connected to your audio interface. If you plug a guitar into the left input of an interface with separate left and right inputs then you would have to use the left input in the audio track. If you plug into the right input then that's what you'd use in the audio track.

If you're recording something that is in stereo, like a stereo effects device, synthesizer, or using two microphones and you want both on the same track you can use stereo.

You can also use two tracks to record both Left and Right on their own track. Use two audio tracks and use the Right input on one and the Left input on the other. It's totally up to you how you use these inputs.

Generally, people record stereo tracks only when it is needed or desirable. One reason is that stereo audio tracks are twice as big (file size) as Mono tracks. Also to consider is what kind of processing you will be using on the track. But different people do things different ways. Find what works best for you.