These are described by the Auswärtiges Amt (Foreign Office) as follows:
participation in consultations aimed at determining Germany’s negotiating position, if the Bundesrat would have to be involved in a corresponding domestic-policy measure or if the Länder would be competent nationally;
participation by Länder representatives in negotiations, if essential Länder interests are involved;
transfer of negotiating powers to Länder representatives in the case of EU proposals which affect exclusive Länder legislative powers (i.e. education, cultural affairs, radio and television...