Where can resources for mail processing categories be found?

The most appropriate source for resources related to mail processing categories is found in DMM101. The Domestic Mail Manual (DMM) serves as the primary guide for mail services and provides comprehensive details on various mail classifications, regulations, and processing standards. It is specifically designed for postal employees and serves as a reference point for understanding the different categories of mail, which is crucial for efficient mail processing and customer services.

In contrast, other resources mentioned may cover related topics but do not focus solely on mail processing categories. Publication 52 addresses hazardous, restricted, and perishable items in the mail, while the Postal Explorer is an online platform that provides general information about postal services but may not delve into the specifics of mail processing categories. The Retail Manual, on the other hand, is tailored more towards retail services and customer transactions rather than the detailed regulations and procedures surrounding mail processing. This distinction highlights why DMM101 is the most relevant and authoritative source for this inquiry.