A File refers to a modeling file for the analysis of a mechanical system. A model is composed of subsystem, mesh and part files. These files can have a relationship through a Subsystem Assembly as shown in the figure below. Assembly is possible only in a subsystem. Higher and lower subsystems, meshes and parts in the hierarchy are referred to as parent subsystems and child subsystems, respectively.
The figure below shows good and bad examples of file structure. A subsystem cannot be a child subsystem of itself because this creates a problem of circular reference.