A compilation unit is a unit of VSI  Pascal  code  that  can  be
  compiled independently; the term refers to either a program or a
  module.  Both programs and modules have declaration sections but
  only  programs  have  executable  sections.   A  program  can be
  compiled, linked, and run by itself,  but  a  module  cannot  be
  executed  unless  it  is  linked  with a main program written in
  PASCAL, or another  language.   A  module  may  contain  routine
  declarations  but  these  cannot  be executed independently of a
  program.