| U | Explanation |
| U001 | ACSE/Presentation Services API (XAP) |
| U004 | ACSE/Presentation Services API (XAP) |
| U007 | Message Store API (XMS) |
| U010 | X/Open DCE: Remote Procedure Call |
| U011 | Distributed TP: The TX (Transaction Demarcation) Specification |
| U012 | Remote Operations Service Element (XAP-ROSE) API |
| U017 | DCE 1.1: Time Services Specification |
| U019 | Data Link Provider Interface (DLPI) |
| U023 | Transport Provider Interface (TPI) |
| U037 | Networking Services, Issue 5 |
| U038 | Networking Services, Issue 4 |
| U039 | X/Open Single Sign-On Service (XSSO) - Pluggable Authentication |
| U042 | System Interfaces and Headers, Issue 5 |
| U043 | Commands and Utilities, Issue 5 |
| U045 | Commands and Utilities, Issue 4, Version 2 |
| U048 | Commands and Utilities, Issue 5 |
| U050 | Networking Services (XNS), Issue 5.2 |
| U051 | CDSA/CSSM Authentication: Human Recognition Service (HRS) API |
| U052 | System Interface Definitions, Issue 4, Version 2 |
| U053 | System Interfaces and Headers, Issue 4, Version 2 |
| U054 | System Interface Definitions, Issue 5 |
| U055 | System Interfaces and Headers, Issue 5 - Two Volumes |
| U057 | Single UNIX Specification, Version 3 Technical Corrigendum No. 1 |
| U058 | X/Open Curses, Issue 4, Version 2 |
| U059 | Single UNIX Specification, Version 3 Technical Corrigendum No. 2 |
| U062 | ARM Issue 4.0 V2 - C Binding |
| U063 | ARM Issue 4.0 V2 - Java Binding |
| U064 | Systems Management: Common Management Programming Interface (CMPI) |
| U065 | TOGAF Version 8.1 Enterprise Edition Technical Corrigendum |
| U066 | Systems Management: Common Management Programming Interface (CMPI) |
| U081 | ITSC Conformance Requirements V1.0 |
