20.4.3 System Routine Arguments
  All of the system routine arguments are described in terms of
  the following information:

  .
        OpenVMS usage
  .
        Data type
  .
        Type of access allowed
  .
        Passing mechanism

  OpenVMS usages are data structures that are layered
  on the standard OpenVMS data types. For example, the
  OpenVMS usage mask_longword signifies an unsigned long-
  word integer that is used as a bit mask, and the OpenVMS
  usage floating_point represents any OpenVMS floating-point
  data type. Table 20-4 lists all the OpenVMS usages and the
  BASIC statements you need to implement them.

  If a system routine argument is optional, it will be indicated
  in the format section of the routine description in one of the
  following ways:

        [,optional-argument]
        ,[optional-argument]

  If the comma appears outside the brackets, you must either
  pass a zero by value or use a comma in the argument list as
  a placeholder to indicate the place of the omitted argument.
  If this is the last argument in the list, you must still include
  the comma as a placeholder. If the comma appears inside the
  brackets, you can omit the argument altogether as long as it
  is the last argument in the list.