in file persistent_object.php, class ezcTreePersistentObjectDataStore ezcTreePersistentObjectDataStore is a tree data store that stores persistent objects.
in file persistent_object.php, method ezcTreePersistentObjectDataStore::fetchDataForNodes() This method *tries* to fetch the data for all the nodes in the node list $nodeList and assigns this data to the nodes' 'data' properties.