From what I understand, a COA would most likely be used by a municipality like a FireDepartment or or City EMS division that would have their own program. The program would have standard Ops for training, maintenance, flight as well as documentation. If you are flying under that COA, which would have specific guidelines in the COA, you would not need to be Part 107? Someone please correct me if I am wrong. This was frequently used in the past to allow exception to fly at night for one example. We had one for our SnR group early on.
COA is very general term and can mean different things.
If you go to DroneZone and get an authorize to fly in restricted air space that is a COA, in fact LANNAC is technically a COA.
An organization like a police department can get one as well, and people flying for them may not need a part 107. These are good for 2 years.
This is a COA I got from Drone zone in this case it was an Authorization to fly in restricted the space: