title: DICOM: What's the point of SOPInstanceUID tag?
body: <p>DICOM already provides a unique enough identifier for the Series (e.g. <a href="http://dicom.innolitics.com/ciods/rt-structure-set/rt-series/0020000e" rel="noreferrer"><code>Series Instance UID</code></a>), so why also include one on the lower level objects (e.g. <a href="http://dicom.innolitics.com/ciods/rt-structure-set/sop-common/00080018" rel="noreferrer"><code>SOPInstanceUID</code></a>)?</p>
  
  <p>What I find really annoying is the fact that when referencing other objects - for example when <code>RTPlan</code> object references <code>RTStruct</code> object via <a href="http://dicom.innolitics.com/ciods/rt-plan/rt-general-plan/300c0060" rel="noreferrer"><code>ReferencedStructureSetSequence</code></a> / <code>ReferencedSOPInstanceUID</code> - it's done using the <code>SOP Instance UID</code>.  However any of the DICOM SCPs - such as find/move - don't work with <code>SOP Instance UID</code>, they work with the <code>Series Instance UID</code>.  So what gives?  Do I have to load the whole <code>Series</code> to find all the referenced objects?</p>