What you are seeing with the \\ display in mp3tag is a placeholder to indicate a multi value field. Other programs may indicate this in different ways, such as the semicolon in Musicbrainz you have found.
This can occur in the Composer, Artist, and Genre fields most commonly.
If you are trying to manipulate the line, you may first want to use the MERGE action where you can assign a new separator. This will combine all of the values of the same field name and allow you to make the edits you wish. There is also actions to SPLIT them in a similar fashion.