The surface normal vector can point in the direction external to the material or the direction internal to the material. Both vectors are normal to the surface. The desire to point away from the surface is, in my opinion, simply convention when measuring on a CMM to ensure that the surface is measured on the correct side (the CMM can figure out the appropriate approach).
It is used to define a point on the part, not instructions on how to measure it. The point faces away from the surface, so that's the vector(as A-machine-insp said).
