WHY we need do "Divide the sequence into segments"

Hello everyone.

       Have anybody can tell me WHY we need do "*Divide the sequence 
into segments*" in the ToASCII process  as described in the
IMAA draft (draft-hofmann-imaa-03.txt)??

        And  another question: "The "*protected code points" are 0..40, 
5B..60, 7B..7F (in other words, those corresponding to ASCII characters 
other than letters and digits). *" is from draft-hofmann-imaa-03.txt.  
But the code points of digits range between 0...40,so why said the 
*digits is not  protected code points*?
      
        thanks..

                                                                        
                           Xiaoli Dong
                                                                         
                           CNNIC