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

    Specifies symbols that  are  either  defined  externally  in
    another module or defined globally in the current module.

      .WEAK symbol-list

    symbol-list

    A list of legal symbols separated by commas.

    When .WEAK specifies a symbol that is  not  defined  in  the
    current module,  the  symbol  is externally defined.  If the Linker
    finds the symbol's definition in another module, it uses that
    definition.   If the  Linker  does  not  find an external
    definition, the symbol has a value of 0 and the Linker does not
    report an error.  The Linker  does not  search a library for the
    symbol, but if a module brought in from a library for another
    reason  contains  the  symbol  definition,  the Linker uses it.

    When .WEAK specifies a symbol that is defined in the current
    module, the  symbol  is  considered to be globally defined.
    However, if this module is inserted in an object library, this
    symbol is not  inserted in  the  library's symbol table.
    Consequently, searching the library at link time to resolve this
    symbol does not cause the module  to  be included.