20.1.2 Passing Parameters by Descriptor
  A descriptor is a data structure that contains the address
  of a parameter, along with other information such as the
  parameter's data type and size. When you pass a parame-
  ter by descriptor, the BASIC compiler passes the address of a
  descriptor to the called routine. You usually use descriptors
  to pass parameters that have unknown lengths, such as the
  following:

  .
        Character strings
  .
        Arrays
  .
        Compound data structures

  Like parameters passed by reference, any change made to
  the value of a parameter passed by descriptor is reflected in
  the calling routine.