The term "parameter passing mechanism" refers to the way in  which  data
  is  passed  to  a  subprogram.   BASIC  supports three parameter passing
  mechanisms with the following BY clauses:  BY  VALUE,  BY  REF,  and  BY
  DESC.   By  default,  BASIC  passes  and receives scalar numeric data by
  reference, and strings and arrays by descriptor.  All BASIC FUNCTION and
  SUB subprograms receive parameters BY REF or BY DESC.

  If a program consists entirely of BASIC program modules, you should  not
  specify  any  parameter  passing  mechanism.  However, other programming
  languages may not pass or receive parameters in the same way.   You  may
  have  to  specify  a  parameter passing mechanism when calling non-BASIC
  programs.