PDG Identifiers are digital object identifiers assigned and used by PDG to reference items of PDG data such as particles, particle properties, decay modes and review articles.
Once defined, a PDG Identifier is guaranteed to not change and can thus be used in other systems as a permanent reference to PDG data. Note that although the meaning of a given PDG Identifier will not change, there is no guarantee that the corresponding data will be included into future editions of the Review of Particle Physics
For further information on PDG Identifiers and their use in the PDG API
please see the documentation