§1. The synod of bishops has a permanent general secretariat presided over by a general secretary who is appointed by the Roman Pontiff and assisted by the council of the secretariat. This council consists of bishops, some of whom are elected by the synod of bishops itself according to the norm of special law while others are appointed by the Roman Pontiff. The function of all these ceases when a new general session begins.
§2. Furthermore, for each session of the synod of bishops one or more special secretaries are constituted who are appointed by the Roman Pontiff and remain in the office entrusted to them only until the session of the synod has been completed.
Source: Code of Canon Law (Vatican.va)