The man page for systemctl says that the command `list-units` will "List known units..." and `list-unit-files` will "List installed unit files."

The point of confusion for me, and possibly other users is in the phrases "known units" vs "installed units". Those sound to me like they should be the same thing -- but the commands obviously present different information, and a unit that might show up in the output from one command may not show up in the other.

...I was eventually able to surmise the difference when I reread the description of the `--all` option, which only applies to `list-units` but I tend to read about the commands before I read about the options, so it was not super clear to me before.