DICOM PS3.3 2019b - Information Object Definitions
C.8.8.14.4 Multiple Aperture Blocks

All blocks with Block Type (300A,00F8) of APERTURE for a given beam shall have equal values of Block Transmission (300A,0102) and/or Block Thickness (300A,0100) if they are specified. The composite aperture shall be evaluated as the union of the individual apertures within a single Block. Shielding block transmission(s) shall be applied multiplicatively after the (composite) aperture has been evaluated.

DICOM PS3.3 2019b - Information Object Definitions