The ICAO PBN Panel is currently working on Amendment 13 to Volume 3, which will likely include:
Criteria and requirements for establishing the altitude at or below which the vertical position of an aircraft is controlled by reference to altitudes. International Civil Aviation Organization Implementation and Status icao doc 8168 volume 3
If you need a deeper dive into a specific section of Volume III (e.g., low visibility operations, RNP AR approach procedures, or missed approach gradient calculations), let me know and I can expand that part. The ICAO PBN Panel is currently working on