I created a tag called "Source" to use for documenting the original physical or digital media type (CD, SACD, Vinyl, Cassette, Bluray, 8mm, etc.). Other software uses a Source tag for this purpose as well.
In support of my request to add "Home Audio" as a media subtype, I think it is valid for the same reasons that "Home Video" is. It is not a genre, but a subtype of Audio. Multiple genres may fall under a Home Audio subtype, such as Family, Interview, Record Keeping, Recitals, Journal, and so on.