The surface structure contains data that describes the boundary of the surface, and a pointer to the primitive surface on which it lies. The primitive surface is a three-dimensional geometric surface parameterized by two variables (u and v). The surface boundary consists of closed loops (contours) of edges. Each edge is attached to two surfaces, and each edge contains the u and v values of the portion of the boundary that it forms for both surfaces. Surface boundaries are traversed clockwise around the outside of a surface, so an edge has a direction in each surface with respect to the direction of traversal.

Other data found in the surface structure includes the rectangular extents of the two-dimensional domain, the three-dimensional surface, and a flag indicating whether the surface normal points towards the inside or outside of the part. The user data is intended for run-time use only, and this information is not stored with the surface.

This section describes the surface data structures. The data structures are listed in order of complexity. For ease of use, the alphabetical listing of the data structures is as follows: