This term refers to a tolerance zone which is bounded by two curves.
The area tolerated as the profile forming area must lie between two equidistant enveloping surfaces.
The spacing of these enveloping surfaces is formed by imaginary spheres with the diameter of the specified value, encompassing the entire specified curvature.

Profile of a surface in Injection Molding

Quality assurance: Tolerated surface profile with representation of the enveloping surfaces

Related topics:

This glossary of plastic industry is provided by PLEXPERT Canada Inc.