>of an object are defined. SolidWorks uses a parasolid kernel, as does
>Unigraphics. In a parasolid kernel, complex edges are defined by the
>intersections of surfaces, a 2 dimensional function held to within a
>tolerance governs the accuracy of the curve. Some kernels define
>edges for each surface individually and hold this information
>separately from the definitions of other faces.
Any number of surfaces could have exactly the same edges ...
thus leaving the solid undefined.
Naturally, you can have problems when there are either gaps or
overlaps ... and need a "solid".
Constructive Solid Geometry (CSG) is little used these days but
involved boolian operations on solid primatives, such as spheres
and such, which were closed solids in the first place.
IF you want to learn a little about internal computer representations
study IGES as a beginning. You do, after all, first need to know what
entities are and any system that can read & write IGES files must
also represent the same data in some form of one to one and onto
mapping of data (though often any one system can also add or
manage a lot more data about entities and their relationships than
poor IGES supports).