The 3D ACIS® Modeler (ACIS) is Spatial’s prominent 3D solid modeling engine. 3D InterOp is a CAD data translation framework (Interoperability)

From DocR20

The class interface is a set of C++ classes that are used to define the ACIS model geometry, topology, and other characteristics. The classes may be used by an application to directly interact with ACIS through their public and protected data members and methods (member functions). Refer to the class summaries and reference templates for a complete list of all classes and a detailed description of each.

The class ENTITY is a base class from which many ACIS classes are derived. It implements common data and functionality that is mandatory in all classes that are permanent objects in the model, although ENTITY does not itself represent any specific object. The ATTRIB class, which is derived from ENTITY, is used to derive specific attribute classes for system and user attributes. Refer to the section Attributes for information.

Developers may also derive application-specific classes from the ACIS classes for special purposes. Refer to the ACIS Library Reference for information about deriving classes.