This document provides an overview of identifiers [and their standards and register(s)] relevant for the design of blockchain systems and the interoperability of those systems with non-blockchain systems. The following criteria were used for inclusion of identifiers in this document: — Identifiers (and their standards and registers) issued by the public sector for subjects and objects such as citizen numbers, business registration numbers or land registration numbers; — Identifiers that are internationally recognized and fulfil one of the following criteria: — An identifier is an international standard of an SDO (Standard Development Organization); — An identifier is a de facto standard according to the norms of the industry involved (even if it is not an international standard of an SDO). — Identifiers that have relevance for DLT systems as it fulfils one of the following criteria: — An identifier that has been used without DLT, but has proven to solve the problems of DLT services using DLT; — An identifier that was designed with the usage of DLT in mind from the beginning.