Return true if this object represents a reference to an externally managed array, string or user‐defined object and false otherwise. If this method returns false, calling destroy on this object will release the memory used by the array, string, or used‐defined object as well as any meta‐data directly used by this datum (e.g., length information); otherwise (if this method returns true) calling destroy on this object will release any allocated meta‐data, but will not impact the externally managed array, string, or user‐defined object.