When a subsystem (child subsystem) is assembled into another subsystem (parent subsystem), an entity in the child subsystem can be replaced by another entity in the parent subsystem. This kind of entity in the child subsystem must be defined as an interface entity. The interface option is available for several entities such as a Solid Body (Rigid Body), Function Expression, User Subroutines, Strings, and Arrays. In the case of an interface body, all joints and forces of the interface body with other bodies of the child subsystem are maintained after an interfacing. However, the mass and moment of inertia of the interface body are ignored and are replaced by those of the parent body subsystem.