If SUS exists then why there's also POSIX?

Question

We know that to define whether the system is Unix or not there's a Single UNIX Specification(SUS) which is held by OpenGroup. Then why there's another certification as POSIX? Are these things the same?

Answer 1

SUS is a newer standard that evolved from the older POSIX standards and encompasses them. There are actually many parts to the POSIX standards. So SUS and POSIX are not the same. One is a superset of (most of) the other. I wouldn't be surprised if there were several details of POSIX that have since been superceded or deliberately omitted though.