So...I've been modeling with SolidWorks since 2007. I can't believe that I do not know this one. What is the definition of a face and a surface. It seems that they are a bit similar. I am asking because there are two different constructs in the API. IFace2 and ISurface. If they were not two different things they would not have different interfaces.

Soooo...what's the actual definitions?

Play with "Untrim" and you will see.

"Surface" is the full underlying definition, e.g. plane, sphere, toroid, or B-surface. It includes the full domain of the mathematical definition e.g. unbounded plane or complete sphere.

"Face" is the portion of the surface used in a body, including edges and boundaries. E.g. an edge fillet between perpendicular planar faces is 1/4 of a cylinder, bounded by neighboring faces. Its underlying "surface" is an unbounded cylinder.