Convert Tag -> Tag

I'm baffled as to how "Convert - Tag > Tag" is supposed to work. Could someone post an example (and add it to the documentation)?

See the help:
This converter formats tag fields by other fields content, i.e., this converter can also be used for copying the content of one tag field to another tag field.
You can use placeholders for any tag field and scripting functions as format string to format the content of the field specified.

Works like the "Format value" action. The help shows this example:
"Example: to copy the content of the comment field to the encodedby field, set the field to ENCODEDBY and the format string to %comment%. "

There is also an example in the FAQs: /t/967/1