The External page of the Objects Setting dialog:

An external item is an application-defined reference to something outside the scene file.

General Settings

• Item Type - The application-defined item type.
• File - The application-defined file name.

Other Settings

• Bounding Volume - The following volume will be exported if the Maya object is of the correct type:
  • Sphere - The object must be a Maya sphere.
  • Box - The object must be a Maya cube.
  • Cylinder - The object must be a Maya cylinder.
  • Mesh - The object can be of any type that converts to a mesh (that isn't a sphere, box, or cylinder).