Library /sys$common/syshlp/basichelp.hlb Immediate  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Many BASIC statements are executable in immediate mode.  An immediate mode
  statement  is  a  BASIC  program statement beginning in column 0 without a
  line number.  When you end such a  statement  by  pressing  RETURN,  BASIC
  executes  it immediately.  With immediate mode statements, you can examine
  and change variables in the currently executing program.  Thus,  immediate
  mode  is  a  useful  tool  for program design and debugging.  See the STOP
  statement and the CONTINUE command for more information.

  When a program has reached a STOP statement, immediate mode statements may
  only  reference  variables  in the last compiled program unit, and may not
  reference any new variables.   Otherwise  BASIC  signals  an  error.   The
  following  statements  are  illegal in immediate mode and if executed will
  produce an error:

      COMMON        DATA          DEF           DIM           DECLARE
      END           FUNCTION      MAP           EXIT          ON
      ONERROR       RESUME        RETURN        SUB