DICOM PS3.3 2021d - Information Object Definitions
C.8.8.15.2 Referenced ROI Number

The Structure Set ROI shall be used in the RT Brachy Application Setups Module to describe the 3D coordinates of Accessory Devices, Applicators and Channel Shields, but not individual source positions (see Section C.8.8.15.9 and Section C.8.8.15.10).

DICOM PS3.3 2021d - Information Object Definitions