Face plane - orientation and layer precedence for material

General questions, is the plane for a face always perpendicular to the face normal. I’ve been looking at SUFaceGetPlane as a possible source to determine if a face is reversed. I’m currently use a test for faces with materials on both sides and faces with materials only on the back side.

I import models from various clients into another application and they have multiple layers but are using text descriptions for the layer and not a number. If I have faces on different layers from the group, how do I determine the material precedence to be applied to the face. Is there some hierarchy for listing layers.