Since RT Referenced Study Sequence (3006,0012) is Type 3, the Attribute may be omitted.

If RT Referenced Study Sequence (3006,0012) is present with an Item, for each Item, the SOP Class UID is that of the Study itself, not of the Instances within the Study. The Detached Study Management SOP Class (Retired) ("1.2.840.10008.3.1.2.3.1") may be used in Referenced SOP Class UID (0008,1150) in lieu of there being any other private or standard Study SOP Class defined.