MailStream Plus- What are ‘Origin’ or ‘Entry’ 3-Digit Trays in First-Class, Standard Mail, and Periodicals?

In some mailings, the USPS may allow or may require separate trays to be prepared, regardless of volume, for each 3-digit/scheme processed at the SCF that serves the entry facility. 
Say a Letter mailing is being entered at the SCF for ZIP Code 20151. The SCF that serves that area SCF DULLES VA 201, which handles ZIP Codes 201, 226, 227, and 268 (as-of August 2015).  Per the presort regulations a separate tray may be required for each of those 4 3-digits, regardless of volume of mail in each tray.

For example, here are the DMM references pertaining to a First-Class mailing (references are as-of August 2015)-
DMM (Domestic Mail Manual) 235.6 contains the instructions for preparing First-Class Automation mail. 

6.5 b says for 3-digit trays- 
3-digit/scheme: optional, but required for 3-digit price (150-piece minimum except no minimum for origin or entry 3-digit/scheme); overflow allowed. Make 3-digit scheme trays first, then 3-digit trays; see 1.4g.. For Line 1, use L002, Column B. defines the term 'Origin'-
Origin/optional entry 3-digit(s): the ZIP Code in the delivery address on all pieces begins with one of the 3-digit prefixes processed at the sectional center facility (SCF) in whose service area the mail is verified/entered. Subject to standard, a separation is required for each such 3-digit area regardless of the volume of mail.

235.1.4h defines the preparation rules-
An origin 3-digit (or origin 3-digit scheme) tray contains all mail (regardless of quantity) for a 3-digit ZIP Code (or 3-digit scheme) area processed by the SCF in whose service area the mail is verified. If more than one 3-digit (or 3-digit scheme) area is served, as indicated in L005, a separate tray must be prepared for each. A tray may be prepared for each 3-digit (or 3-digit scheme) area served by the SCF/plant where mail is entered (if that is different from the SCF/plant serving the Post Office where the mail is verified). In all cases, only one less-than-full tray may be prepared for each 3-digit (or 3-digit scheme) area.

FYI- If the USPS wants and approves you to turn 3-digit origin trays off, you can do so by putting an N in position 66 of the ENT PT parameter.
UPDATED:  April 21, 2017