Resolve MailStream Plus- Why is it correct for some tray, sack, or pallet levels to contain mail to more than a single 5-digit or 3-digit destination?

MailStream Plus, all versions, all platforms,
MailStream Plus presorts the mail per USPS regulations.  Presorting means combining mailpieces into groups for the various USPS Facilities that handle various ZIP Codes, and combining ZIP Codes together for other similar USPS processing reasons.  
These preparation regulations are defined in the USPS Domestic Mail Manual (DMM), and per the monthly USPS Labeling Lists.

Most of the tray/sack/pallet levels can normally contain mail pieces to more than just a single 5-digit or 3-digit.
Although there are too many levels of trays/sacks/pallets to list individually, below are some common levels that can (and most often will) have mail pieces to multiple ZIP Codes in them-
Any destination level that had the word scheme, such as 5-digit scheme (5DGS), or 3-digit scheme (3DGS)
Any AADC and ADC level.
Any SCF destination level.
Any Mixed AADC and Mixed ADC level.
Any NDC or Mixed NDC level.
Any working (WKG) level.

For further information please refer to the USPS DMM at-

There are lists in the DMM describing what the various tray/sack levels contain.  Here are the current locations for some common types of mailings:
For First-Class- Letters/Cards see 235 1.3.1.  For Flats see
For USPS Marketing Mail- Letters see 345 1.3.1.  For Flats see 245 1.3.2
For Periodicals see 207 18.3
For Bound Printed Matter see 265 1.4 

The USPS website address and DMM references above are as-of February 2017
UPDATED:  April 21, 2017