Wheelchair Mounts H3D

When should I use a Rehadapt H3D Wheelchair Mount?  The H3 D series are hybrid mounts built using 22mm tube at the base level and 16mm tube at the upper level.  These are remarkably strong with the 22 mm tube at the base and, to the user, appear much lighter because what the user sees is a 16 mm tube.  Ideal for a iPad mount for direct select in situations where you need something a little more robust than a mount bullt entirely using 16mm tube..