ANSWER
Both CBN and PCBN relate to Boron Nitride (BN) - a polymorph material formed by two chemical elements. Boron Nitride exists in different crystal structures. One is cubic and the BN in this structure is Cubic Boron Nitride (CBN).
As a cutting tool material, CBN is used as a polycrystalline compound, where CBN particles and an added binder are sintered together. The material produced is "Polycrystalline CBN" or simply "PCBN". The percentage of CBN can vary in different PCBN grades. In the context of cutting tools, the commonly used abbreviations "CBN" and "PCBN" may be considered as synonyms.