If you walk around any urban area while looking at your feet, before long you will (a) bump into a solid object, and, more relevantly for the purposes of this question, (b) see the above legend on a brass plates near the foot of buildings.
This has always puzzled me. They are usually attached to what looks like a concrete drain cover, or sometimes a glass-block skylight set-up. Either way, they don’t look like they’re going to be letting anything out.
So what’s the deal? If the basement is on fire, do they magically open up? Do the fire brigade lever them up to let the smoke out? There never seems to be an obvious handle to lift anything up.