Vatican Treaty (バチカン条約, Bachikan Jōyaku?) is an international agreement in the Rebuild of Evangelion film series that limits the indiscriminate usage of Evangelions.
As far it is known, it allows only three active Evangelion units per country. As a result, upon NERV HQ's reception of Evangelion Unit-03 from America for testing, Unit-02's core is placed into containment under the jurisdiction of the IPEA, being the one of the three currently active Evangelions that technically remains under the control of Europe and not Japan.
Another term in the Vatican Treaty states that a country must not modify standard Evangelion Units for military purposes. This was mentioned by Ritsuko when she first encounters multiple Evangelion Mark.44B and a Evangelion Mark.4444C in Paris in the film Evangelion: 3.0+1.0 Thrice Upon A Time.