The POSIX.1 standard (FIPS 151-2/P1003.1) does provide some security functionality. The security functionality supported includes Discretionary Access Control using a permission bit mechanism and Privilege using a privilege mechanism. It was the intent of the POSIX.6 standard to extend this functionality and add areas of functionality not addressed by the POSIX.1 standard. An application that is POSIX.1 compliant should run successfully on a POSIX.6 compliant system.
The POSIX.1 standard supplies only a subset of the functionality supplied by the POSIX.6 standard. The functionality provided by the POSIX.1 standard should be the minimal acceptable requirement of security functionality for any multi-user system requiring POSIX-like interfaces. Whether the extended and additional POSIX.6 functionality is needed should be determined based on the security requirements of the system.