VMS Help MACRO, VAX MACRO Assembler, Directives, .WARN  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Causes the assembler to display a warning message on the terminal
    or in the batch log file, and in the listing file.

           .WARN [expression] ;comment

    expression

    An expression whose value is  displayed  when  .WARN  is
    encountered during assembly.

    ;comment

    A  comment  that  is  displayed  when  .WARN  is  encountered
    during assembly.  The comment must be preceded by a semicolon.

    If .WARN is included in a macro library,  end  the  comment  with
    an additional semicolon.  If you omit the semicolon, the comment
    will be stripped from the directive and will not be displayed when
    the  macro is called.