I haven't seen this addressed yet... These symbols on seat maps viewed by gate agents mean:

Crown = Sky Priority passenger (GM/PM/DM anywhere, or a first class passenger)
$ = Delta Comfort+ seat
* = Non-rev standby
P = Preferred seat (aisle/window near the front of main cabin)

I'm assuming the Red Coat was viewing a seat map with these same symbols that are shown at gate/ticket counter computers.

There are also symbols for passengers who are arriving from a connecting flight (optional), on a through flight (optional), have a wheelchair request, pet-in-cabin, service animal, etc.