Cluster Key:

The cluster key is the column, or set of columns, in which the clustered tables have in common. You can specify the columns of the cluster key when creating the cluster. You consequently specify the similar columns when creating each table added to the cluster.

For each column specified as category of the cluster key (when establishing the cluster), each table created in the cluster must have a column which matches the size and kind of the column in the cluster key. There is no more than 16 columns can form the cluster key and a cluster key value cannot exceed roughly one-half (minus some overhead) the available data space in a data block. A cluster key cannot involve a LONG RAW or LONG column.

You could update the data values in clustered columns of a table. Thus, since the placement of data depends on the cluster key it changing the cluster key for a row might cause Oracle to physically relocate the row. Thus, columns which are updated frequent are not good candidates for the cluster key.