An extrusion is a solid volume in 3D space, defined by a "base" face (IFMEFace) and an extrusion vector. This solid is defined by the prism generated by extruding the face in the direction and magnitude described by the extrusion vector.
For example, a triangle face, extruded in the direction of the surface normal of that face, yields an IFMEExtrusion that represents a triangular prism.
A positive extrusion vector produces a solid whose orientation is front facing out, with all surface normals facing out. A negative extrusion vector extrudes in the direction away from the surface normal of the base face, and produces a solid whose orientation is front facing in, with all surface normals facing in.