As defined by the Smithsonian national Postal Museum, “Auxiliary markings are notations made on an envelope regarding services to be performed and status of the mail piece within the postal network. They may be hand-written comments, hand-stamped, or applied labels.”
I have written an article about these markings, which can be found in Forerunners, the Journal of the Philatelic Society of Greater Southern Africa (PSGSA), Issue #113, September-December 2025. This article was honoured with the 2025 Article of Distinction Award by the PSGSA. A copy of the certificate can be found here.
References
Marking Devices. Smithsonian National Postal Museum. Retrieved from https://postalmuseum.si.edu/exhibition/about-postal-operations-mail-processing/marking-devices on September 27, 2025.