There's another category I would add to Phil's list:

d) All songs sung by a Source Singer (or a Well Established Folk Singer)

This has become particularly apparent to me, as I've been listening to a lot of the Musical Traditions label albums recently (all crazily cheap to download currently).

It's interesting that all the songs get Roud Index numbers, including ones that were sentimental tunes of the 20s and 30s popularised by Classical singers or crooners or country singers - songs that the Source Singer would have learned from a 78rpm or radio.

One case in point being the song 'City of Laughter, City of Tears', which I heard recently on the Bob Hart album 'A Broadside'. Thanks to youTube, I can hear that this is a Light Classical piece recorded by the tenor William Thomas, with orchestra.

There's a corollary to this Stateside, in the case of singers like Mance Lipscomb or Mississippi John Hurt, where the boundary between folk song and jazz/blues standards and country standards of the day gets blurred.