What are the differences between POSIX, the Single UNIX Specification, and the Open Group Base Specifications? I think their purpose is for determining if an OS is Unix?
One important "difference" is the XSI (XOPEN) option group, which specifies a number of traditional Unix interfaces which are unlikely to be applicable to a new operating system that's not "a Unix". Often people will use "SUS" to refer to the full standard including the XSI option, and "POSIX" to refer to the base standard and possibly the non-Unix-centric option groups.
Successive versions of SUS seem to be gradually deprecating parts of the XSI option that were not really useful for writing portable applications and that instead just reflected legacy implementations, and moving a few of the more-useful parts to the base standard.
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