Query on draft-ietf-sip-body-handling-04- Can required multipart/mixed contain required body part

Hi folks,
 
sec 7.2 of draft-ietf-sip-body-handling-04
 
      Also per the previous rules, a 'multipart/mixed' body whose
      handling is required can contain only body parts whose handling is
      optional.
 
I wonder why required multipart/mixed can NOT contain required body
part?
I think it's just same meaning repeated once, should be fine.
 
any thoughts?
 
thanks
-Rockson

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