Is the posix namespace used?

The C++11 standard §17.6.4.2.2 defines posix as a reserved namespace for use by ISO/IEC 9945 and other POSIX standards, and states that C++ programs adding declarations or definitions to the posix namespace have undefined behaviour.

Is that namespace actually used by ISO/IEC 9945 or any other POSIX standards? If yes, please provide an (online) reference, if possible.