What is an "A1 Bescheinigung" and when do I need it?
Due to European social security regulations, Freie Universität Berlin employees who are conducting business travel must carry with them a so-called A1 certificate (the official German name is “Bescheinigung über die anzuwendenden Rechtsvorschriften – A1”), regardless of how long the business trip lasts. This applies for business travel outside Germany to countries who are members of the European Union and also Iceland, Liechtenstein, Norway (EEA countries) and Switzerland.
Please read more here.