The A1 document certifies which social security legislation applies to the holder of the form. It is generally required in situations where an employed or self-occupied person has a connection through his/her work activity with more than one European Union (EU) country. Under the EU rules a person is only subject to the legislation of one country at any one time.
The A1 certificate is issued by the country to whose legislation you are subject and confirms that you are not subject to the legislation of any other country you are connected with. In other words you will have no obligation to pay social security, including health insurance, contributions in the other countries. The A1 certificate remains valid until the date of expiry, indicated at the form or until it is withdrawn by the issuing institution.