Explaining why MailStream Plus changes the first 2 bytes of the IMb when presorting Flats

MailStream Plus™, all versions. all platforms.
There are many USPS sites that discuss the format of IMb, for example-

And this site says that the first 2 bytes of the IMb is called the 'Barcode Identifier' and gives the possible values.

(URLs checked as of Dec 2018)

The first link above says this about the Barcode Identifier field- 
...The Barcode Identifier field should be “00” (zero-zero) with one exception: automation-price eligible flat mail bearing a printed optional endorsement line (OEL). When mailers prepare flat-size pieces using IMb tracking codes to meet automation-price eligibility requirements, the IMb tracking codes on any pieces bearing printed OELs must contain the Barcode Identifier corresponding to the printed OEL used. See the Barcode ID Reference Table on PostalPro to determine the correct Barcode Identifier

Note it says ...should be “00” (zero-zero) with one exception: automation-price eligible flat mail bearing a printed optional endorsement line (OEL). 

But MailStream Plus™ (MSP) will always change the Barcode Identifier to reflect the Bundle or Tray/Sack level the mail piece is in.  It does not matter whether the piece is Automation or not, or gets an OEL (Optional Endorsement Line) or not. 

Pitney Bowes consulted with the USPS, and while the references above talk about auto vs non-auto, the main thing is that all Flats bundles, whether auto or non-auto, currently either get an OEL printed on them or they get Facing Slips or Stickers.  
The USPS said that they are okay with the correct Barcode ID going into the IMb for both auto and non-auto flats. They said the only time they *actually* need the 00 to be left as the Barcode ID is when the ‘stickers’ are being used. 

Some Mailers pass-in the IMb to be used in the Name-Address input records (using IMBOUT position 11=P).  As discussed above, based on the presort level, MSP will update the Barcode Identifier field in the records, therefore Mailers should use the 'updated' IMb's that come out in the presorted Name-Address output files on the mailpieces.  
And the updated IMb's are what will be put into the Mail.dat PDR or PBC files that MSP produces. 
UPDATED:  March 12, 2019