Final answer:
The appurtenance used to indicate subsequent awards of most US military decorations is an oak leaf cluster, which is attached to the ribbon of the original medal.
Step-by-step explanation:
The appurtenance used to denote the award of second and subsequent awards of decorations (other than the Air Medal), the Army Reserve Components Achievement medal, the National Defense Service Medal, and unit awards is referred to as an oak leaf cluster in the United States military. These are small bronze or silver pins attached to the ribbon of a medal. For instance, if a person were to receive the National Defense Service Medal twice, they would wear the original medal along with one bronze oak leaf cluster on the ribbon to indicate the second award. When someone receives a sixth award, a silver oak leaf cluster, which represents five awards, is worn in lieu of five bronze clusters.