Communication protocol between pilots of aircraft and air traffic control are based on a document from ICAO published in Annex 10 Volume 2 (Communications Procedures) to the Convention on International Civil Aviation and ICAO PANS-ATM (Procedures for Air Navigation Services - Air Traffic Management) Doc. 4444.
Frequencies are a limited resource in the aviation radio spectrum as voice communications are handled between 117.975 - 137 MHz. In the busy European airspace the current and expected demand are very high for communication channels. Therefore VHF 8.33 kHz channel spacing was introduced in 1999 for airspace above FL245.
After this the European Commission went on to regulate airspace down to FL195 in regulation No 1265/2007 and then airspace below FL 195 in regulation No 1079/2012. This came into effect Jan 1st, 2018. From this moment on all aircraft in European airspace need to be equipped with 8.33 kHz channel radio's.
These operators also had to have their radio's upgraded with the new channels with regard to their use of VHF frequencies.
Excerpt from the document:
During the last 10 years military authorities have made significant efforts to migrate towards VHF 8.33 kHz channel spacing. More than 1000 State aircraft were equipped. Based on information provided by the States to the EUROCONTROL in 2016, the 8.33 kHz equipage rates of the European State aircraft fleet in 2020 will be around 88% for transport type State aircraft, 78% for fighters and 68% for light aircraft/trainers.
To continue reading see the pdf below: