When a class is clustered, it is physically reordered based on the index information. The clustering is static. In other words, if the class is updated, it may become unclustered. No attempt is made to keep new instances or updated tuples clustered. If desired, the user can recluster manually by issuing the command again.
The table is actually copied to temporary table in index order, then renamed back to the original name. For this reason, all grant permissions and other indexes are lost when cluster is performed.
cluster emp_ind on emp