DICOM PS3.3 2019c - Information Object Definitions
C.8.8.14.15 Source to External Contour Distance and External Contour Entry Point

The Source to External Contour Distance (300A,0132) is the distance to the beam entry point (External Contour Entry Point), which may include Bolus, Patient Positioning Devices, Patient Immobilization Devices or other devices. This value is useful for including the attenuation effects of external devices on the dose calculation and for patient setup.

DICOM PS3.3 2019c - Information Object Definitions