DICOM: What's the point of SOPInstanceUID tag?

Question

DICOM already provides a unique enough identifier for the Series (e.g. Series Instance UID), so why also include one on the lower level objects (e.g. SOPInstanceUID)?

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Answer 1

Series reference is not specific enough. In the case of structure sets the Reference SOP Instance UID ties the contours in the structure set to the specific slice in the dataset. It's not enough to just reference the series because you have to ensure that the contour is exactly aligning with a slice.