The Vatican Treaty (バチカン条約) is an international agreement in the Rebuild continuity allowing only three active Evangelion units per country. As a result, upon Nerv HQ's reception of Evangelion Unit-03 from America for testing, Eva-02 is placed into containment under the jurisdiction of the IPEA, being the one of the three currently active Evas that technically remains under the control of Europe and not Japan.
The treaty's ostensible reason for existing could be simple politics, in order to prevent any one country from amassing too many of the most potent weapon on Earth. However, given that a policy restricting Japan's access to Evangelions is wholly counterproductive in light of their position as the world's defense against the Angel threat, Seele would probably have been able to pull some strings were there not a "deeper" reason for the three-Eva limit. One reasonable speculation that has been forwarded: four Evas active at any one place and time would risk recreating the conditions of Second Impact, by assembling the equivalent of four Adams.