Title should be "USPS adds a line to its operating manual clarifying what a postmark is." And reading between the lines USPS is telling you (and everyone else,

Title should be "USPS adds a line to its operating manual clarifying what a postmark is." And reading between the lines USPS is telling you (and everyone else, including other government agencies) that the postmark is for their internal business use and you should not try to derive any meaning from it.