Historical use of them quoted from the Table Asia Gallery:
Ranma (also spelled Rama or Ramma) transom were used in Japanese architecture, because the typical room dividing fusuma or shoji doors are under six feet in height, there is space in between the top of the sliding door and ceiling. Ranma, (literally: space for ran, a type of wood) or transom panels, are used to artistically fill those spaces. They also serve the practical purpose of allowing ventilation and light into the interior of a house or temple. Ranma can be carved three dimensional pictorial scenes, geometric slatted wood designs, or natural wood slices of trees.I included the Japanese version below in case you can read it and because you can make the details out much easier.
Generally, the more intricately carved and three dimensional, the older the ranma transom. Ranma transom can have black lacquered wood frames or a natural finish.