What is the purpose of Field in replace action

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.

Also in case you missed it: http://help.mp3tag.de/options_format.html

It explains clearly what each function does.

A lot of the confusion for me is about the language used. For Format value, it reads:

Formats tag fields by other fields content, i.e., this action can also be used for copying the content of one tag field to another tag field.

Which is about copying content to tags. The casual reader would not think Format value had anything to do with copying or tags, since neither word appears in the action name.

Lots of things don't strike me as intuitive. I want to remove a tag, so now that I know about the Format value action, I thought I could format the tag into nothingness. But if you try that and select the field (aka 'tag') you can't leave the Format string blank (why not?). Instead you need to use a different Action type (Remove fields). But wait, earlier I was Formatting values, and now I'm Removing fields. So are values, tags, and fields all referring to the same thing? Appears to be.

mp3tag is a powerful tool. I get that and see why it is popular. But the grouping of operations seems peculiar to me. I would have had it set anything (filename, tags) based on anything (filename, tags, nothingness) and let the source, destination, and formatting be expressed in regexs (with tons of keywords), instead of Convert and multiple Action subcategories.