VMS Help SET, PREFIX, Parameter  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 string

    Specifies the FAO control string to be used in generating a
    prefix to a verified command line. The following rules apply:

    o  No more than 64 characters are allowed in the control string.

    o  The resulting string can be no longer than 64 characters.

    o  Basic formatting FAO directives can be used ("!/", "!_", "!^",
       "!!", "!%F", and "!n*c").

    o  Time and date FAO directives can be used ("!%T" and "!%D").

    o  Repeat counts can be used ("!n(DD)").

    o  Output field length specifications can be used ("!lengthDD").

    o  Combination of repeat count and output field length can be
       used ("!n(lengthDD)").

    o  FAO directives that require arguments will always receive a
       value of zero.