Hey there geetarr people!

I've been trying to play Stop This Train (John Mayer) for a while and all well, but there's something that still bugs me when I listen to the track.

There's this percussive flick/slap pattern, but I can figure out whether the 'slap-sound' comes from the flicking nail when he slap-flicks, or because the slap cause the string to hit the fretboard? It doesn't sound the same each time if you listen closely. Also, when do you consider it a 'slap' instead of just playing the string?

I know it's a rather nitpicking question, but still, I'd be happy for any input