I've been asking about this over at the Steinberg user forum for over a year with no response at all. So I figured I'd try it here just in case there someone around who knows Cubase and wants to shine a light. The thread has three messages, all by me. They represent the history of what I managed to figure out working by myself over the course of a year.

Here's the thread, and thanks in advance...
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The Cubase 9 Operations Manual has a chapter on Tempo Matching Audio starting on page 488 (EDIT: This is from a year ago. In the Cubase 9.5 Operations Manual, the page number is 475).

I imported some audio, and went through the procedure described on pages 492 and 493 (EDIT: the 9.5 manual page numbers are 479 and 480), lining up all bars and beats as the instructions specify. I thought that by doing this, I could then get the audio to play at a steady tempo, and to play at whatever tempo was set in the project. And as part of what I thought I could accomplish, I figured that when I was done the audio would play in sync with the Cubase metronome.

I noticed that the procedure moved bar and beat markers on the lower of the two rulers in the Sample Editor. The upper ruler stayed exactly the same. And, surprisingly to me, the Cubase metronome always stayed in sync with the upper ruler. In other words, all the bar and beat adjusting I did as I followed the procedure had absolutely no effect on the Cubase metronome.

Meanwhile, there's another procedure in the manual starting on page 929 (EDIT: the 9.5 manual page number is 899) which shows you how to do Tempo Detection. I successfully used this method to tempo-match my imported audio to my project.

All well and good, but it leaves me wondering what the point of the first procedure is. As far as I can tell, all I accomplished with the first procedure was to line up bars and beats on the Sample Editor's lower ruler, but that ruler does not seem to affect anything. Can someone help me understand what the point of that first procedure is, and how to use the results of it?
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I finally figured out what the deal is with the Sample Editor and the two rulers. The top ruler is just the time layout of the project. The bottom ruler is the “definition” of the audio file, i.e. where the bars and beats are.

The main thing that was messing up all my experiments was that I didn’t understand the difference between Musical Time Base (a track property), and Musical Mode (a property of a particular segment of audio). As a result, sometimes I was setting MTB when I really needed to set MM, and as a result I wasn’t getting consistent results and couldn’t figure out what was going on.

Now I can see that the two procedures are just different ways to set up the “definition” for the audio.
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I just looked into this some more, and it seems that the two procedures are entirely separate from each other. I now understand how to use either of them. But I'm wondering why there are two of them.

Does anyone know? Are there some things that one is better for than the other?