It’s unclear why the Armed Forces of the United States chose the oak leaf cluster to represent the subsequent issuance of an award already presented to a service member. The simplest explanation—that oak trees symbolize strength, endurance, and faith—is also the most plausible, but the fact remains that none of the applicable military manuals explain the reasoning behind the selection.
How the oak clusters are to be worn, however, is spelled out in considerable detail. A single bronze oak cluster is awarded for each subsequent award of a type already earned by a service member; a silver oak leaf cluster is worn in place of five bronze oak leaf clusters. Also, no more than four oak leaf clusters, regardless of their color, may be worn on a single ribbon; in case where adding another cluster violates this rule, a second ribbon is instead placed beside the first one. When the number of clusters is reduced to four or less (through the replacement of multiple bronze clusters with silver), the second ribbon is removed.
For instance, imagine a service member who has earned nine Joint Meritorious Unit Awards. The first is represented by the ribbon itself; the second through six are represented by a silver oak leaf cluster, while the seventh, eighth, and ninth awards are signified by bronze oak leaf clusters, adding up to four clusters in all. When the service member is presented with the tenth JMUA, a single bronze oak cluster would place five clusters on the ribbon, which is not allowed, not would it create enough bronze oak clusters to be replaced a single silver cluster. Therefore, the existing ribbon configuration would remain unchanged, and a second ribbon would be placed beside it (to the wearer’s left).