Plone support for z3c.relationfield. If this package is installed, you should be able to use z3c.relationfield as per its documentation for Dexterity and Archetypes content.
Note that this package does not depend on Dexterity or Archetypes directly. However, if plone.dexterity is installed, the DexterityContent base class will be marked with z3c.relationfield's IHasRelations. Similarly, if Products.Archetypes is installed, its BaseObject class will be marked with IHasIncomingRelations, thus allowing relationships to Archetypes objects from Dexterity content.
Other types of content can be supported by appropriate use of these marker interfaces. See z3c.relationfield for more details.