CEVAP:

Ad hoc diplomacy is the oldest form of diplomacy that aims to conduct diplomatic relations by sending a special and/or temporary envoy on mission. It was the common practice of diplomacy
during the Middle Ages in Europe and during the classical age of the Ottoman Empire. Although
diplomatic relations today are mostly conducted through permanent missions, in some cases, states might still prefer sending special envoys, usually to conduct diplomatic negotiations on specific issues or create a non-permanent representation or an additional channel for diplomacy with a state or government.