The surfaces, not their edges, are the bounding elements. 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).