Markers (https://docs.oracle.com/cd/E24152_01/Platform.10-1/ATGCommStoreGuide/html/s1401usingorderandcommerceitemmarkers01.html) provide the ability to tag (almost arbitrary) extra data against Orders and Commerce Items.
Note, in all my years working with ATG, I have never come across their use.
Is it possible to use the presence of a marker as a qualifiers for promotions?