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  1.4.5  implementation-defined behavior            [defns.impl.defined]
  behavior,  for  a well-formed program construct and correct data, that
  depends on the implementation and that each implementation shall docu-
  ment.

  1.4.12  undefined behavior                           [defns.undefined]
  behavior, such as might arise upon use of an  erroneous  program  con-
  struct  or  of  erroneous  data,  for  which  the  Standard imposes no
  requirements.  Undefined behavior may also be expected when the  stan-
  dard  omits  the  description  of any explicit definition of behavior.
  [Note: permissible undefined behavior ranges from ignoring the  situa-
  tion  completely with unpredictable results, to behaving during trans-
  lation or program execution in a documented manner  characteristic  of
  the  environment  (with  or  without  the  issuance  of  a  diagnostic
  message), to terminating a translation or execution (with the issuance
  of a diagnostic message).  Note that many erroneous program constructs
  do not engender undefined behavior; they are required to be diagnosed.
  ]

  1.4.13  unspecified behavior                       [defns.unspecified]
  behavior,  for  a well-formed program construct and correct data, that
  depends on the implementation.  The implementation is not required  to
  document which behavior occurs.  [Note: usually, the range of possible
  behaviors is delineated by the Standard.  ]

Regards,
Bengt Richter