Is Posix a subset of Single UNIX Specification? I know a question about the differences between Posix and SUS has already been asked and answered beautifully. Anyway, the answers seemed to suggest the possibility that SUS "encompasses more than Posix", and there are certain things in SUS that are not included in Posix. An answer specifically addressed the XSI (XOPEN) option group as the only difference, but added that SUS seems to not care so much about it anymore. Now I'm wondering if there is any other difference, or they are just named differently for historical reasons? Moreover, wikipedia seems to suggest that there is a difference and that Posix is the core of SUS : > Very few BSD and Linux-based operating systems are submitted for compliance with the Single UNIX Specification, although system developers generally aim for compliance with POSIX standards, which form the core of the Single UNIX Specification.

There is no other difference.

The SUSv4, 2016 edition site states that it is

> Technically identical to IEEE Std 1003.1, 2016 Edition and ISO/IEC 9945:2009 including ISO/IEC 9945:2009/Cor 1:2013(E) and ISO/IEC 9945:2009/Cor 2:2017(E) with the addition of X/Open Curses.

IEEE Std 1003.1 is POSIX.

You can also verify this by looking at the table of contents: XBD, XSH, XCU, and XRAT are the four sections of POSIX, leaving only XCURSES in SUSv4 but not in POSIX. All of POSIX is in SUSv4, so POSIX is a subset of SUSv4.