This thread and the one it referenced prompted me to poke around. In the other thread, Kirk mentioned that:
The Catalog system can only perform queries on images that it knows about. Images that are not known to the catalog are not in its database and cannot be queried.
The color of the dots (the catalog status indicator) vary between:
Unfilled (background color): completely unknown to any catalog.
Yellow: known to a catalog but currently offline or missing.
Green: known to a catalog and currently online.
I went to my Apple Photos Library in the navigator (folder browser/favorites view not the catalog view -- by the way, I think that the catalog viewer and folder/favorites browser have somewhat confusing non-intuitive icons), and opened the originals folder and it's subfolders in a new contact sheet. Many of the images had green dots. However, All of the images have Apple's naming convention of a GUID.extension, so they look something like 00B431CA-D5F7-49DA-B55D-F26FE09038F6.jpeg.
Some of the images in my Apple Photos library have been added to at least one of my PM+ catalogs, but with their original filenames, (which Apple stores in it's SQLite DB and cross references it with the GUID.extension filename stored in the originals folder). What's confusing me is that as far as I could remember, I had NOT added the apple photos library folders to any of my catalogs.
However, I just checked, and I did, in fact add that folder to it's own catalog at one point. So that could explain the green dots. Except that catalog was not enabled for either search or add/modify when I navigated to the Apple Photos Library originals folder. But the PM+ contact sheet still seemed to know that I had added some of the images to a catalog at some point. even though it wasn't "enabled" at that time. Is this how it's supposed to work? I suppose if I had PM+ "forget" that catalog, then it wouldn't display the green dots. Is that correct?
From some other threads, I can actually see how it would be useful, while using the navigator and contact sheets to browse a directory hierarchy, to be able to sort by the dot (catalog) status, and even be able to have the catalogs an image has been added to be displayed somehow (and their online/offilne status per catalog because that could be different on a per catalog basis), perhaps as a variable or some other way.