A cluster is a schema object that contains data from one or more tables, which have one or more columns in common. Oracle Database stores together all the rows from all the tables that share the same cluster key.
PowerDesigner models clusters as extended objects with a stereotype of <<Cluster>>.
Clusters in Oracle v10gR2 and earlier are modeled as indexes with the Cluster check box selected. To upgrade such clusters to v11 or higher, you must generate a new PDM with the appropriate DBMS target from your original model. Simply changing the target DBMS will result in the loss of any existing clusters