You could try an Internet search to learn about iTunes tagging, but I believe that not all fields (tags) are stored in the file. People sometimes get around this by making playlists of files that have certain tags, exporting those from iTunes and then importing into MC. It's been discussed on the forum, probably on the Mac board.
You can add any tag to MC as a custom tag, and if the tag exists in the file, the contents will be seen in MC, but the name must match exactly.
Sorry: MC=PMW for this purpose.