Authority involves the right to be obeyed and the duty of obedience by those subject to authority. The French government claimed the right to exclude Greenpeace from Muraora atoll. Normally we seek to act in accordance with the best reasons which apply in a particular case.
Authoritative commands are exclusionary reasons which pre-empt further deliberation.
Legitimacy also refers to specific ties binding authority to recipients. R. P. Wolff claims that the acceptance of authority is incompatible with the autonomy that is a necessary condition for moral agency. Moral agent acts on balance of reasons and does not surrender right to decide. This is not a specifically Kantian problem since it applies to act-utilitarians too. The Raz-Green response Acceptance of authority gives people better chance of acting on the reasons which apply to them.