ImageDM: what does extension mean?

[Ray Plante]

Hi DMers,

If you were at the IVOA Interop meeting, then you know that image cube 
access was a big topic, and the ImageDM is one of the key standards 
targetted to support it.  We also resolved to get greater 
participation in and discussion of these specs going forward, and I 
was hoping I might inspire a bit of that with a question about 
ImageDM.

At the Interop, there was a resolution to make the ImageDM be an 
extension of ObsCore.  The purpose of this is to provide greater 
interoperability SIAv2 and ObsTAP:  that is, you might discover data 
cubes with either one, and you're query results would be based on a 
common data model.

My question is, what does it mean technically for ImageDM to "extend" 
ObsCore?  

One result we might expect is that the ImageDM would not be explicitly 
"re-defining" the ObsCore metadata and UTypes; rather it would point 
to the ObsCore document for its definition.  Is there more to it, 
though?  Is there something implied about the structure of UML models?  
How about serializations?  Should a client that is only ObsCore-aware 
be able to interpret the ObsCore portion of an ImageDM serialization?  
What does it mean for how we define the extension metadata?  

I'd like to hear people's take on this as we begin revisions of the 
ImageDM document. 

thanks,
Ray