Auto Calibration Setting

This support article describes it in more detail than the manual: https://support.garmin.com/en-US/?faq=FhOYuggxmV6Atph276U4h8

That being said I don’t think the article will provide the details you’re asking for. And I’m not sure if these details have been provided by Garmin at all.

As JJ wrote, Garmin doesn't provide enough detail for any consumer to provide a precise explanation.

My interpretation, partly from JJ's link is, "Continuous Calibration uses a combination of GPS, DEM (Digital Elevation Model), and user calibration information." Just how this is done, isn't described.

I have a 66i, not a 67i, but I don't get an Altitude plot unless I start recording an activity. This is how the device knows you've started an activity.

I think periodic is a better descriptor than continuous, but the period isn't defined.

I don't think Auto Cal overrides manual cal. A manual cal is used as a baseline. If you're inputting a manual cal, you wouldn't set Auto Cal to once, because you are inputting a known value which is better than a calculated value. With Auto Cal set to Continuous, the algorithm would use the manual cal as an input, see Garmin quote above, and re-calibrate periodically.

I would love to see a detailed explanation of all aspects of the altimeter, the upper and lower plots, what the different colours mean on the plots, and everything mentioned above. Garmin manuals are sad, you learn much more from chat rooms and online forums than their crummy manuals and it’s been this way since the beginning. What a great opportunity for someone to make an extensive Garmin for Dummies book, but how can you write a book when no information is available. I’ve been in this game for awhile, and am still learning things, I feel for the technically challenged people, they must be totally lost after purchasing a Garmin.

Yes, Garmin documentation lacks detail and can be wrong. I've also been in this game for a while and continue to learn.

The upper plot represents the entire activity and the bottom plot is a zoom view of a portion of the upper plot. On my device, the lower plot is a light green, and the upper plot is a dark green except for the zoomed portion, which is a light green.

I can scroll over the plots and get elevations as I scroll along.

For planning purposes I prefer to use Google Earth, where I can see the track and the elevation changes on one page. Once out in the field, the Altimeter is pretty useful.

Looking at product documentation from Garmin’s point of view, its fitness products don’t need much explanation.  Easy experiments usually tell the user what he needs to know, and tech support is just a phone call away.  On the other hand, people entering environments where nav and com problems are critical and need to be solvable without support are different.  Pilots low on fuel in instrument conditions can’t rely on Garmin tech support, and experimentation is out of the question.  So government regulators require clear documentation, and regulators require that pilots understand it. InReach products are an odd man out.  They’re designed for folks like pilots who are leaving the Internet and cell service and going into an environment where navigation and communication might be critical.  But no public authority requires good documentation.  That’s the bad news for those of us who rely on InReach devices.  The good news, is that the cost of those devices is a tiny fraction of what comparable Garmin aviation products cost, and when things go wrong in the backcountry, we usually have more time to work around the problem than pilots have.  All that said, I too wish I could find out how my 67i works.