For the purpose of ascertaining whether issuance of an order is approved by producers, importers, and in the case of an order assessing handlers, handlers, the Secretary shall conduct a referendum among producers, importers, and, in the case of an order assessing handlers, handlers, not exempt under section 4606(e)(4) of this title, that, during a representative period determined by the Secretary, have been engaged in the production, importation, or handling of honey or honey products.