A transom is the lite, or glass, that is over the top of the storefront door. If
you have side lites then the transom can span over each side lite, one side lite
or just over the door.
The dimensions for a transom are variable in width and up to 48" in height.
The most common reason to have a transom above your storefront door is to allow
light from the outside / exterior to shine into your location. This light is often
referred to as the "daylight opening", meaning, how much of the daylight from the
exterior side will be allowed to shine into the interior.