In this area and I imagine most Exit signs are required to be illuminated.
To that end they are either suppled from an emergency generator circuit and will likely be tied into the local emergency lighting circuit.
They have battery back up either self contained or remote.
Once an exit sign is backed up with a battery it becomes "Unit Equipment" and the rules of 700.12(E) apply.
700.12(E) is long but requires unit equipment to be supplied by the local lighting circuit.
part of 700.12(E)
The branch circuit feeding the unit equipment shall be the same branch circuit as that serving the normal lighting in the area and connected ahead of any local switches.
If you read all the way to the end of 700.12(E) you will see this exception that sometimes allows a dedicated circuit to supply unit equipment.
Exception: In a separate and uninterrupted area supplied by a minimum of three normal lighting circuits, a separate branch circuit for unit equipment shall be permitted if it originates from the same panelboard as that of the normal lighting circuits and is provided with a lock-on feature.