The Euro, which is the official currency of the European Union. It is a requirement to eventually adopt the Euro if you join the EU; however, two countries (Denmark and the United Kingdom) have opt-outs, meaning they are not required to adopt the Euro.

The Euro is the currency of the European Union. All members are obliged to eventually switch to the Euro in a reasonable amount of time (unless they've achieved opt-outs, which only the UK and Denmark have). This is so there can be easy trade within the European Union, as there is only one currency.

There are several members of the European Union who do not use the Euro. However, all but two of these countries have noted that they will switch to the Euro in some time, as this is the law of the European Union. The two countries to not do so are the United Kingdom and Denmark. Both have opt-outs on the situation and are not obliged to adopt the Euro. However, this doesn't mean they won't do so in the future, as it is possible.

Originally the European Union was a treaty organization for the purpose of doing away with tariffs on coal and steal. It added other commodities. Gradually, it became a confederation with suggested laws for all its members. Some taxes applied to some members and some to all members. It is slowly evolving toward a Federation. Certain decisions taken in Belgium apply to all members.

The European Union is not a complete federal union where the decisions made at governmental headquarters apply to all members. It is still partly a confederation where states can opt out of certain decisions. As in a confederation, they can not opt out of the decisions they have permanently turned over to the supranational government.

Yes, you'll have to write a letter explaining your request to opt out of Union support. Keep in mind though you will not have a union to back you up when needed.