Thanks for the feedback. But perhaps the FLAC comment name should be source, and MP3 tag name should be target?
Okay, so I created a column named MediaType, with Value and Field both set to %mediatype%. Next, I mapped VorbisComment SOURCEMEDIA to MEDIATYPE. Is this correct?
Previously, I also created columns for TrackTotal (Value/Field %tracktotal%), DiscNumber (Value/Field %discnumber%), and DiscTotal (Value/Field %disctotal%). Was that correct, and should I also add mappings for these?
So far, all I've been able to find regarding Vorbis comment standards are all quite old:
 Ogg Vorbis I format specification: comment field and header specification
Appears to be an official page, only date is: © 1994 - 2005
 Field names
Also appears to be an official page, last edited on 18 August 2015
 Proposals for extending Ogg Vorbis comments
by David on Sep 28th, 2004
 Ogg Vorbis Comment Field Recommendations
by Jonathan Walther, Last updated Thu Jul 3 20:35:53 PDT 2003
The xiph documentation  states that field names are to be: "case-insensitive field name that may consist of ASCII 0x20 through 0x7D, 0x3D ('=') excluded." So, ASCII only. This also specifically states that field names may contain spaces (ASCII 0x20). Yet every one of the standard field names on that same page, is without spaces (eg. TRACKNUMBER).
The proposed additions to the standard field names, also from xiph  are also without spaces (eg. TRACKTOTAL, DISCNUMBER, DISCTOTAL), and include SOURCEMEDIA, as proposed by Jonathan . But David  seems to take exception, proposing 'SOURCE MEDIUM', saying "My final gripe is that the author of the recommendations [referring to Jonathan ] seems to have an irrational fear of spaces. Spaces are explicitly allowed in field names and should be used when appropriate instead of creating jumbled messes of letters."
On this I side with the official xiph documentation, as none of their names include spaces. However, I have to agree with David  that " The concept of singleton tags, though logical for some fields, drastically limits the flexibility of the comments when applied to AUTHOR."
Apart from those minor issues, both  and  contain valuable extensions to the very limited standard lists of  and . I wonder if we'll ever see an official extended list?