Library /sys$common/syshlp/nclhelp.hlb set, x25_access, template, target_address_extension  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Default: No NSAP address                      Value: NSAP address

    Called NSAP address to be sent to the called DTE should be
    supplied by a higher-level entity in the calling DTE. A null
    address indicates that this facility is not required.