I found this when I was looking for information on
M-bags:
Printed matter is admissible in M-bags. Printed matter is defined as paper on which words, letters, characters, figures, images, or any combination thereof, not having the character of a bill or statement of account, or of actual or personal correspondence, have been reproduced by any process other than handwriting or typewriting. Articles that meet the printed matter definition include newspapers, magazines, journals, books, sheet music, catalogues, directories, commercial advertising, and promotional matter.
(And this is one of the more comprehensible things written by the US government or wholly-owned subsidiary thereof.)