Models of STEP:

a)  Form-feature model describes the object from the point of view of manufacturability whereby the form pattern is described or a method to produce the form element. The manufacturing relevant features can include holes, threads.

b)  Shape representation model is based on the elements of geometry and topology model. There are three methods of object representation specified; the wire frame, surface, and volume models. In the volume we can have constructive solid geometry, and boundary representation. The concept. in these models is the formation of a neutral shell over the basics elements of the topology model. The intention of this concept is the separation of the operation models from the topology.

c)  Topology model defines the entities for description of neighbourhood relations. Surfaces, edges and vertices are defined. These topological features are described by association with corresponding geometric.

d)  Geometry model contains all geometric information about the definition of lines, and surfaces. By specific entity structures, points, vectors, coordinate systems, transformations, curves and surfaces can be described. Free-form surfaces can be approximated to computable forms.