you could call it N too. W shows up in GHpython I think.
but the point is less about what to call it. there are other situations in which a relative Z shows up, like anytime you pick a SubD face with the gumball on. It seems consistent for that little UV indicator to have a W or an N. The only way it could tell you which face of the surface is front and which is back without a third arrow is if it appears to be mirror imaged when you look at it from the backface