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.

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    Downvoted without explanation, a clear, on-topic, research made and shown, question. No words – Gabriele Scarlatti Jan 25 '18 at 8:41
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    Have an upvote to balance things out. To be fair, your question as given in the title was already answered by the answers to the existing question; the really new question here (“if there is any other difference”) doesn’t jump out ;-). – Stephen Kitt Jan 25 '18 at 8:53

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.

  • POSIX is a group of standards. It is only POSIX.1 which is a proper subset of SUS. – fpmurphy Sep 5 '18 at 13:52
  • @fpmurphy1 that used to be the case, but POSIX.1, .2 etc. were merged into a single standard starting in 2001; at first the rationale was left out but the 2008 revision merged that. – Stephen Kitt Sep 5 '18 at 14:01

In former times, POSIX was not a complete UNIX standard but started in 1988 with a description on what is in libc only.

In 1992, a description on on commands has been added but it was too basic that people did not like it.

Since there was a need by e.g. the government to have a better standard, the single UNIX specification started around 1995 (UNIX-95) as an "enhancement" to POSIX and defined what people needed, e.g. the demand that a process needs to be able to be at least in more than one group.

Later IEEE reduced their work on POSIX and since SUS Issue 7 (IIRC), IEEE stopped their own work completely. Since then, a IEEE standard typically is release 6-12 months after a SUS standard after some internal voting in the IEEE commitee.

So today, SUS is the basic standard and IEEE is bound to that.

  • UNIX 95 is a brand, not a specification. A brand and a specification are two different things. – fpmurphy Sep 5 '18 at 13:58
  • You are wrong, UNIX 95 is a synomym for the SUSv1 standard. – schily Sep 5 '18 at 14:07
  • @Schilly. The Single UNIX Specification was the core of the UNIX 95 product standard and brand. See opengroup.org/openbrand/register/xum4.html. UNIX 95 and SUS (there never was a v1) are definately not the same thing. – fpmurphy Sep 5 '18 at 14:46
  • Well in 1998 there was a SUSv2, to the first one could be called SUSv1 and in fact is usually called this way by the creators of that standard. – schily Sep 5 '18 at 14:52
  • @Schilly. You still do not get it. SUS is not a standard, it is specification and, no, the creators of the original version of the SUS did not and do not not call it SUSv1 – fpmurphy Sep 5 '18 at 15:03

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