Thanks for the additional information. ... I think...
As a result, I have just spent an hour or two learning about junctions, hard links, and soft links.
My conclusion is that for my needs (linking on a single computer, not across the local net) the following commands are functionally
identical:
(command line from anywhere as long as junction.exe is in C:\system32)
junction.exe "C:\Program Files\Calibre2" "D:\Win_10_apps\Calibre2"
(command line from within C:\Program Files)
mklink /D Calibre2 D:\Win_10_apps\Calibre2
Is there any advantage in mklink on a single computer? I will use mklink in the future because it is an internal command, but is there any point in going back on the junctions I have already created with junction.exe?