A compilation unit is a block of code that can be compiled  as  a  unit.
  The  compilation  unit  is the initial token displayed on your screen by
  the Language-Sensitive Editor.  Before you start your  editing  session,
  you  must  select  the  type  of  program  unit you plan to edit.  BASIC
  compilation  units  include  PROGRAM  units,  and   SUB   and   FUNCTION
  subprograms.