A Cluster is composed of two or more ComputerSystems, operating together. A ComputerSystem may participate in multiple Clusters. ...

A Cluster is composed of two or more ComputerSystems, operating together. A ComputerSystem may participate in multiple Clusters. 

When first establishing or bringing up a Cluster, only one ComputerSystem may be defined as participating in it. Therfore, the cardinality of the association for the ComputerSystem reference is Min(1).