USPS Marketing Mail is mail matter not required to be sent as "First-Class Mail" or "Periodicals". Essentially, synonomous with bulk mail.
" Mailers use USPS Marketing Mail to send:
• Printed matter, flyers, circulars, advertising.
• Newsletters, bulletins, and catalogs.
• Small parcels."
See "USPS Bulk Mail". https://pe.usps.com/BusinessMail101?ViewName=StandardMail