DICOM defines three different hierarchies for image-related Query/Retrieve operations, each of which has different semantics and allowable search levels.  The allowable combinations of QueryRoot and QueryLevel are:

In addition, to the above values, QueryRoot may be set to “WORKLIST” to indicate a Modality Worklist query, or “GP-WORKLIST” to indicate a general purpose worklist query, but if this is done, then a DoRawQuery must be performed, as the parameters for the query DataSet are very different to those used for the other queries.

DicomQuery Object  | DicomQuery Members