Seems to be mostly down to interpretation. On one hand the glib answer would be the clue is the term 'POP' but in recent years in Folk Clubs I've heard songs my The Kinks, because many of their songs were about the London 60's fashion & culture be somewhat Folkyfied, in the folk sense of music being of the times and culture. perform a lot of Trad Irish songs, where trad usually related to old songs by anonymous writers. But again in rcent year songs snch as The Town I loved So Well, The Rare Auld Times, even Dirty Old town ( not even Irish or Anon) become regularly listed and referred to as trad. So is it down to age of the song or definition, or perhaps where, when and how it's sung??