DICOM PS3.3 2021e - Information Object Definitions
A.6.4.2.2 Active Image Area Overlays

The Overlay Plane Module may be used to describe the active image area by use of an overlay. In this case, an Overlay Type (60xx,0040) value of R and an appropriate Overlay Subtype (60xx,0045) value is used.

DICOM PS3.3 2021e - Information Object Definitions