It’s been a while since I’ve gotten in the lore of Halo but my guess would be “hey passing by or living in proximity to this particular massive life destroyer 9000 ring is ok because this one has fired and cleared it’s part of the galaxy/system”. Probably a way to categorise which areas have had their rings triggered for traversal or flood surveillance. Like if three rings fired and each one was successively closer to a forerunner installation it could potentially be a form of early warning. Since all the Halos can be accessed/fired remotely from the Ark which was a forerunner repository of all sentient life. That said, rule of cool is a thing and delta halo does sound cooler than 04.