The "Protocol on Asylum for Nationals of Member States of the European Union", known colloquially as the "Aznar Protocol" after former Spanish prime minister José Maria Aznar, restricts, although doesn't completely eliminate, the possibility of a member state of the European Union offering political asylum to a national of another member state, as would be the case of Carles Puigdemont if he requests protection from Belgium.
The protocol is an annex to the Treaty of Amsterdam, which came into effect in 1999. It argues that states are members of the EU because they comply with certain high standards of democracy and respect for fundamental rights, which would be incompatible with one of their citizens becoming a refugee. To be able to request political asylum, an individual must be persecuted based on their ethnicity, race, religion or political ideas without the state or the judicial system able or wanting to do anything with regards to the matter.
Specifically, the protocol has a single article which establishes that "given the level of protection of fundamental rights and freedoms by the Member States of the European Union, Member States shall be regarded as constituting safe countries of origin in respect of each other for all legal and practical purposes in relation to asylum matters."
Aznar was the main architect of the protocol, not wanting ETA terrorists or sympathisers to enjoy political asylum within another member state. The article ended up gaining four exceptions, however, which Belgium could now use to consider a hypothetical request from Puigdemont.
The first is when a state has suspended the European Convention for the Protection of Human Rights and Fundamental Freedoms within its territory, as they can do "in time of war or other public emergency" according to article 15 of the Convention.
The second is when a procedure against the state has been initiated for breach of the Union's fundamental values, pending a final decision on the accusations.
The third is when such a procedure has led to the Council suspending the member state.
The fourth of the exceptions proved by the protocol reads: "if a Member State should so decide unilaterally in respect of the application of a national of another Member State; in that case the Council shall be immediately informed; the application shall be dealt with on the basis of the presumption that it is manifestly unfounded without affecting in any way, whatever the cases may be, the decision-making power of the Member State."
It should be noted that Belgium submitted a declaration which the conference took note of stating that it would "carry out an individual examination of any asylum request made by a national of another Member State". This position was confirmed by a civil servant from the Belgian Embassy in Ottawa and one from the European Asylum Support Office (EASO) in a document Spanish news agency Europa Press has had access to. In this case such a request could be considered. According to EASO, "every adult that requests asylum from the EU in Belgium is invited to an interview and, in principle, a justified decision is taken".
Belgium has granted asylum to other Europeans before. The document, dated 2015, says that the country received 220 requests from EU nationals, mainly from Romania, Slovenia, Bulgaria and Croatia. In 2013, all such petitions were rejected, but in 2014 10 were accepted.