Cornflour is an ambiguous regional label. In British and many Commonwealth recipes it usually means cornstarch, the fine white starch used for thickening. In other contexts, corn flour means finely ground whole corn or maize flour.
Check the package ingredients and the recipe's origin. Starch and whole-grain flour behave differently in sauces and baking. Neither name by itself means masa harina, which is made from nixtamalized corn. This entry keeps the meanings separate rather than redirecting every use to one ingredient.
Reference: Further reading.