Same on both the Mac and Windows versions of Mp3tag:
I start with populated metadata for Artist, Album and Title
Then when I do a Discogs search - based on Artist and Album - it finds one or more matches
I select one of the matches
In macOS (only):
Click [Merge] (no equivalent on the Windows variant)
The title gets overwritten - typically with that of a different track (of same album)
Other already-populated fields are not overwritten (AFAICT)
In Windows:
AFAICT there is no Merge option so it can only overwrite everything.
Is there a way to prevent the overwriting in Mp3tag-macOS > Merge ?
Is there a Merge in the Windows version (that I haven't found), or is one planned?
Just for you to comment on whether I got it right, and to document what worked (for any others or myself in years to come) what I did was as follows...
Yes, Save replaces the tag with the fields enabled in the Tag Sources results window, Merge combines the existing tag with the fields enabled in the results window (overwriting any existing contents).