GAFilk's secret guest this year, Emma Bull, collects found filk. I believe this can be defined as "filk that does not come from the filking community". This includes stuff like the songs from the musical episode of Buffy, some of Jonathan Coulton's songs, modern cowboy songs... quite a lot of material. And, since Emma asked us to send found filk her way, I asked for a definition of "Not-Filk".
She said that this was an interesting question.
mneme defined it as "mimetic". He said that "Title of the Song" is found filk, but the songs it spoofs are Not-Filk.
Emma noted that Stan Roger's "White Collar Holler" is found filk. Logically, most of his songs should be Not-Filk, such as the wonderful "45 Years From Now", one of my favorite love songs, but we sing it at filks, so it is filk.
Musicals fall into an interesting slot. I think the songs from The King and I are Not-Filk. I think. osewalrus, mnemex, and I sang "Sit Down, You're Rocking the Boat" as the dead dog filk wrapped. Does that make it filk? What about the songs from Rocky Horror, Sweeney Todd, and Urinetown?
And, what about the songs from Grease? At first glance, these look like Not-Filk. But, as those who've seen the stage play and those with a really quick ear who've seen the movie know, there are at least two songs one could make an argument for being found filk. One of these is the school anthem for Rydell High School. The other is the parody of this anthem that the kids sing in the halls of that venerable institution.
Categories may well be fictions, but they are often useful fictions. However, they are more useful when they are exclusive. The problem I've had with the term "interstitial" is that I cannot figure out what is not interstitial. I want to send Emma cool found filk, but this is much harder if I don't know what not to send her.