The processing category for USPS
https://faq.usps.com/s/article/What-is-the-Non-Machinable-Surcharge-for-First-Class-Mail
Large envelopes, newsletters, and magazines
https://pe.usps.com/BusinessMail101?ViewName=Flats
Letter packaging
https://pe.usps.com/BusinessMail101?ViewName=Letters
A mailpiece that can be sorted within of the standard, automated mail process
https://pe.usps.com/text/qsg300/q201.htm
A mailpiece that must be sorted outside of the standard, automated mail process
The processing category for USPS
https://faq.usps.com/s/article/What-is-the-Non-Machinable-Surcharge-for-First-Class-Mail