2.1.1347 Part 4 Section, anchor (Anchor Point)

a.   The standard implies that the anchor element anchors the backdrop plane.

This element specifies a point in 3D space that is the offset to the origin of the backdrop plane.  By moving this anchor point, the image of shadows, glows, and reflections can be moved and can be rotated from its default position on a plane parallel to the x-y plane to any location and orientation in 3D space, providing it remains viewable in the viewing frustum of the scene camera.