Introduction to draft Supplement 186 (reproduced from the document for your convenience)

This Supplement adds a new extensible IOD and SOP Class that allows new Content Item types to be used as they are added to the standard.
There is an increasing need for new Content Item types (such as SCOORD3D) and these are being added as needed to application-specific IODs and SOP Classes (such as the Colon CAD Supplement 126). There is a need for a generic IOD and SOP Class to allow new applications to make use of these in a general way, yet this conflicts with the requirement to be able to render all content.
It is also possible that new Relationship Types will be added, and the IOD also addresses these.
A specific conformance requirement is specified, that the user be warned if there is content of a type that the rendering software does not recognize or understand.
This is consistent with the approach used in other extensible formats like PDF, in which software tools like Acrobat Reader warn about unrecognized content when faced with newer versions.

Download links:
Sup 186 PDF Document - https://standards.nema.org/kws/groups/09-DICOM-2/download/12757/latest/sup186_lb_extensiblesr.pdf

Sup 186 XML format - https://standards.nema.org/kws/groups/09-DICOM-2/download/12758/latest/sup186_lb_extensiblesr.xml