If the tax authority signature is registered in the argentine consulate then the argentine consulate will certify it. Get it certified by a notary public. The tax affidavit is valid for 15 months, after said period of time a new one must be issued in case of a port call after the expiring date. FYG, in the event that one of the vessels included in a valid tax affidavit already sent a local agent comes to Argentina, and is attended by another agent, a legalized copy can be handed over to the new agent, saving you the hassle of sending a new one. They are often needed in adoptions, extraditions and certain business transactions.
Authentications are most often part of a chain of proof that involves a number of cover documents.
To simplify this, the Hague convention established a system that allows the state secretaries of state to be the last officials in the chain. The apostille for Hague convention countries is simply a special form of authentication.