for flight service DFW-ORD-LHR, the aircraft for the DFW-ORD portion only has an
economy cabin, so premium service cannot be offered. For example, a premium seat with extra leg
room or entertainment equipment that is usually available in a higher cabin. However, the ORD-LHR
portion has first, business, and economy cabins. To offer a brand for the second portion of the service,
you can specify secondary RBDs. The following example will search for the primary RBD initially,
then if it is not offered, the processing logic will check the secondary RBDs.