Using a Fence in 3D
When you place a fence in a 3D file, it is planar and parallel to the view in which it is placed. A fence encloses the volume bounded by the area of the fence and the view's Display Depth — that is, the volume enclosed by moving the fence from the front to the rear of the view volume along the view z-axis.
Even if an element is completely within a fence's boundaries, if it also intersects the view's front or back clipping plane, it is considered to cross the fence and is clipped if Fence Mode is set to Clip.
This is important to remember, in particular, when creating 3D cells.