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.