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Mailstream Plus does the 3-digit or 3-digit Scheme tray for First-Class letters have a USPS rule for one before the other?

Is there a USPS rule that says whether the 3-digit or 3-digit Scheme trays for First-Class Automation letters must come first?

No, there is not a USPS rule to say which must comes first.  Because MailStream Plus is a software program, internally MailStream Plus presents the 3-digit trays and then the 3-digit Scheme tray for First-Class Automation letters.
 

UPDATED:  April 18, 2017