VERIFIED SOLUTION i

Using Sort Field 008 to put heaviest trays, sacks, bundles on pallets first in MailStream Plus

Product affected: MailStream Plus™, all versions, all platforms.
With some exceptions, in general the USPS requires heaviest trays to be on the bottom of pallets (for safety reasons, so the pallets are not 'top-heavy' if fuller trays are placed on top.)

In MailStream Plus™ (MSP), Sort Field 008 is a choice available on SRTFLx parameters. And is also used as a sort field value in all the SRTFLD parameter selections (except for SRTFLD 0004 and SRTFLD 0009). 
Sort Field 008 causes Trays to go on pallets by weight, heaviest first.

Note: the USPS rule is about Trays on pallets, and due to the way 'sacks on pallets' and 'bundles on pallets' are prepared there is not the same 'top-heavy' concern for them.  However- using 008 will also have MSP put Sacks on Pallets, and Bundles on Pallets, heaviest first too.
UPDATED:  May 20, 2019