The Convention provides a Contracting State with alternatives
It can declare that a category of NCRIs in the form of rights of arrest or detention under its laws to secure payment of amounts due for such public services as airport landing fees or air navigation charges or other airport charges in respect of an aircraft specifically has priority under paragraph 1(a) of Article 39(1), and it can declare under paragraph 1(b) that it preserves the right to arrest or detain an aircraft for failure to pay for such public services provided to an aircraft. If the Contracting State declared and applied paragraph 1(a) it could do so only with respect to non-consensual, and not with respect to any contractual, rights or interests permitting it