I'm untangling this suggestion now. It's pretty deep into the architecture and I will have a better understanding after I dig into the docs and come up for air.
Anyone wanting to follow me down the rabbit hole:
https://docs.plone.org/external/plone.app.dexterity/docs/reference/misc.html
http://plone.293351.n2.nabble.com/plone-autoform-why-use-tagged-values-td7560956.html
I have to read @optilude threads very slowly and carefully, but I always come out the other side quite enlightened.