When and for which countries do CAU employees need an A1 certificate?

As a general rule, every employment abroad (business trip/secondment) to an EU/EFTA member state (Iceland, Liechtenstein, Norway and Switzerland) must be accompanied by an A1 certificate. Any border crossing for professional reasons requires an A1 certificate.  

CAU employees who, at the employer's instruction, are staying in an (EU) foreign country or an EFTA state for temporary work (e.g. as part of business trips, conference participation, further training, meetings, etc.) must have an A1 certificate in good time before the start of the trip  (cf..  Decree of the Ministry of Finance of 11.06.2019,(german only)).
In order to enable processing and timely forwarding to the DLZP, applications must be submitted to the Human Resources Division of the CAU at least three weeks before the start of the trip.