The distinction is entirely based on product form: haddock that has been separated into boneless or skin-on muscle blocks (fillets) is classified under the fillet provisions of heading 0304, while whole dressed fish, cheeks, collars, and other non-fillet cuts are classified here under 0304530020. The physical state of the product at the time of importation — not the intended end use — determines the correct line.

Yes — haddock is a species with documented mislabeling risk in the seafood trade, where lower-value whitefish species are sometimes substituted. CBP and FDA have authority to require species verification, and FSIS may conduct species testing during reinspection. Importers should request species certificates from the exporting establishment and ensure commercial invoices use both the common name and scientific name (Melanogrammus aeglefinus) to reduce examination risk.