Focusing on a single market where goods, people, services and movement of capital by free. The country has an economic Union and integration (17 from 27 countries) will use the single currency is the euro.
Focus on a single market for the moving party. Goods, services and capital freely. Countries with economic and monetary union (17 of 27 countries) to use a single currency is the euro.
Focusing on a single market for transferring individuals. Products, services and capital freely. Countries with economic and financial integration is the Union (17 from 27 country) will use the single currency is the euro.