VERIFIED SOLUTION i

Why does MailStream Plus place heavier trays on the bottom of pallets, *except* for some, such as MAAD trays.

MailStream Plus™ all versions, all platforms
DMM 705 8.5.6 e and h say basically the same thing-
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e. Heavier, fuller trays must be placed at the bottom of the load, unless other standards (such as 245.7.7) require placement of certain trays on the top of the pallet.

h. Heavier, fuller trays must be placed at the bottom of the load, unless excepted by other standards (such as 245.7.7) that may require placement on the top of the pallet.
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For example DMM 245 7.7 referred to above says-
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Upon presentation of letter-size automation price USPS Marketing Mail mailings to USPS for verification, mailers must present all mixed AADC trays together, and such trays must either be adjacent to one another or side by side, and must be placed as the top layer(s) on any given container.
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Although the line above specifically mentions letters in trays, we believe it applies to flats in trays too.  The intention of putting such trays on the top level of pallets is so the USPS will be able to easily remove the MAAD trays from the pallets for further processing.
The MailStream Plus™ code internally adjusts the values for the 008 'invert thickness' sort field to make this happen for tray levels such as MAAD.

DMM references are as of May 2019
 
UPDATED:  May 16, 2019