I won't bother everybody with my initial post which went deep down into the REPLACE action and much confusion.
Bottom line is that if you want to COPY a tag to another tag, you create an Action type of Format value
Then you select the Tag you want to change by choosing the appropriate Field from the pull-down menu.
That done, you put into the Format string area (in my case) one of the Tag names (delimited with % on both ends) found in the left hand column of the list at http://help.mp3tag.de/main_tags.html (column header is Name in Mp3tag).
When first approaching this issue, I thought copying would involve replacing tags (e.g. replace ARTIST with ALBUM ARTIST) and therefore explored the REPLACE action by mistake. I never would have thought of formatting. But that's how it is.