I am seeing in various places in the API documentation entities called U and V. An example of this is IFace2.IGetUVBounds. It returns the minimum and maximum of U and V for the face. Another method is IFace2.GetClosestPointOn. It returns the closest point in X, Y and Z space and also returns U and V.

I have found some tutorials from other cad software packages that look like U and V are a projection from some sort of flat plane. I don't want to assume though. I have also found some definitions of surfaces as a function of X, Y and Z based on a U, V value.

What is U and V?