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

SOP Instance UID : Represent your a unique Identifier for IOD, Its a TYPE 1 tag must present with value.

For Example :

Each DICOM Image has unique identifier