How to understand a 'batch' and whether Mailpieces are ever moved from one batch to another in MailStream Plus

All versions of MailStream Plus on all platforms
Each 'batch' in MailStream Plus (MSP) is a separate mailing, and must meet it own qualification minimums, and is presorted separately for bundles, trays, pallets, and other preparation settings. 

MSP never moves mail from one batch to another batch.
The 'B CODE' parameter tells MSP to look in a certain location/length in the input Name-Address records.  As the records are read in, any records that have the same value are put into the same mailing 'batch', and presorted separately.
For example, this means that if a pallet minimum is 18 trays, and one batch has 17 trays at an entry point going to a pallet destination, and another batch at the same entry point has one tray to the same destination, then no pallets will be made, because neither batch meets the pallet minimum.

There is one minor specific exception to the general rule that each batch must qualify for the USPS minimums separately.  The use of this parameter option may or may not be acceptable to the USPS in a specific mailing, and should be checked with the USPS before implementing.  
The ML MIN parameter position 32 can be set to Y to *qualify* less-than-minimum *non-automation* mail across separate batches to count as a qualifying total of non-automation mail.  But the batches still are not actually physically combined, nor are pieces moved from one batch to another batch. Please refer to the MSP Reference Guide for additional information.
UPDATED:  October 30, 2017