Complex sentence: As such a senior rank, it is very seldom held, either in a ceremonial capacity by a king or senior member of a royal familiy, or by a very senior air force officer in a large air force.
Split into simpler sentences: As such a senior rank, it is very seldom held. It is awarded either in a ceremonial capacity to a king or senior member of a royal familiy, or to a very senior air force officer in a large air force.