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.