I've added an extra field via a behavior, according to the subject field of the categorization behavior https://github.com/plone/plone.app.dexterity/blob/master/plone/app/dexterity/behaviors/metadata.py#L104, to a type.
How would I limit the selection to just one value?
'maximumSelectionLength': 1 and
'selectionAdapter': 'SingleSelection' as widget directive, but no success. (and yes, the options are visible in