Library /sys$common/syshlp/ncphelp.hlb SET, MODULE, X25-SERVER, KNOWN DESTINATIONS, EXTENSION MASK hex-string  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 Provides a mask for the called address extension facility. Using
 logical AND, the mask is merged with the called address extension
 in  the incoming call and the result is compared with the
 extension value. By default no extension mask is used.