See also corresponding RFC for X509 Internet implementation of certificates  and x509 discussion

X509 is a subset of X500 standards maintained by ITU. It concerns certificates and Public Keys Infrastructure. ISO maintains the standard under its own numbering (ISO/IEC 9594-8, 14888 part 1 , to part 3)

 From ITU, or ISO

Information technology - Open Systems Interconnection - The Directory: Public-key and attribute certificate frameworks, (ISO/IEC 9594-8-2001, FDIS9594-8).  

Information technology -- Security techniques -- Digital signatures with appendix -- Part 1: General, ISO/IEC 14888-1.

Information technology -- Security techniques -- Digital signatures with appendix -- Part 2: Identity-based mechanisms, ISO/IEC 14888-2-1999  

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Information technology -- Security techniques -- Digital signatures with appendix -- Part 3: Certificate-based mechanismsm ISO/IEC 14888-3 and release notes.

Certificate management for financial services -- Part 1: Public key certificates ISO/IEC 15782-1

Banking -- Certificate management -- Part 2: Certificate extensions ISO/IEC 15782-2

Information technology -- Security techniques -- Specification of TTP services to support the application of digital signatures, ISO/IEC 15945.

Health informatics -- Public key infrastructure -- Part 1: Overview of digital certificate services ISO/IEC 17090-1

Health informatics -- Certificate profile ISO/IEC 17090-2, and draft

Health informatics -- Policy management of certification authority ISO/IEC 17090-3