Sorry for the tardy response.

I chose to create a structure specialization rather than force structure into either reference or concept. If that's not an option for you, then I would pick reference because structure information provides details about something that is concrete rather than cerebral.

Creating the specialization allowed me to introduce the semantics of part/description table into DITA. I did this by specializing simpletable and its descendants. If I had to do it again, I would  add support for an alternate presentation by defining a specialization of section called partdescription; the idea being that one would either use a part/description table or a series of partdescription sections but never a mixture.

Regards,

Bob Thomas
Tagsmiths, LLC