Defines the passing mechanism for an external routine. The
    following list describes the passing mechanisms.

    o  BY DESCRIPTOR

       Allows passing character data with any parameter access mode
       to routines compiled by language compilers that implement the
       OpenVMS calling standard.

    o  BY LENGTH

       The LENGTH passing mechanism is the same as the DESCRIPTOR
       passing mechanism.

    o  BY REFERENCE

       Allows passing data with any parameter access mode as a
       reference to the actual data.

       This is the default passing mechanism for parameters. This
       is also the default passing mechanism for a function value
       returning character data.

    o  BY VALUE

       Allows passing data with the IN parameter access mode to a
       routine as a value and allows functions to return a value.

       This is the default passing mechanism for a function value
       returning noncharacter data.