Library /sys$common/syshlp/basichelp.hlb LSE Topics, Compilation unit, PROGRAM unit  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  A main program unit can be compiled, linked, and executed  by  itself,
  whereas a SUB or FUNCTION program unit cannot be executed unless it is
  linked with a main program unit.  A main program unit can be delimited
  with the PROGRAM and END PROGRAM statements; however, these statements
  are optional.