It is possible to manually assign any value to a clusterId element on the condition that this value is the same for all the maps to be clustered.
The clusterId element is case-sensitive. If the case does not match, the cluster will not work.
The following lines show how to set the value Foo for the clusterId element associated with a map:
<?xml version='1.0' encoding='utf-8'?>
<controlFile>
<resources>
<resource>
<filePath>path_to_the_document</filePath>
<clusterId>Foo</clusterId>
</resource>
</resources>
</controlFile>