A 3D Surface is a plane surface defined by 3 or 4 points.
The management of the list of 3D surfaces in the model is done by selecting the option 3D Surface of the Surface menu in the Geometry menu.
The dialog box allows creating a new 3D Surface (New button), modifying an existing 3D Surface (Edit button) or deleting an existing 3D Surface (Delete button).
For defining a new 3D surface you must define the identifier of the object (integer different than zero and different than the identifier of any existing 3D Surface) and you must define the points (3 or 4) that define the 3D Surface.
Modifying an existing 3D Surface can be done by means of selecting the Edit button of the List of 3D Surfaces dialog box. The only non-modifiable field is the identifier of the object. The rest of fields of the 3D surface can be modified as when creating a new 3D Surface.