DICOM PS3.3 2019c - Information Object Definitions
C.8.8.5.4 SOP Class UID in RT Referenced Study Sequence

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, the SOP Class UID of the Detached Study Management SOP Class (Retired) may be used in Referenced SOP Class UID (0008,1150).

DICOM PS3.3 2019c - Information Object Definitions