Seeing it now 👀
What I could be missing then is the ability within a relay to 'sub-relay' in so a manner that a dedicated hardware could give service to different purposes...
As I pointed with the 'house' and the 'region' relay, these already are, —in the sense of the nostr day-to-day experience—, alike theforest's and yabu's respectively: theforest's Stella house where friends and relatives are welcome; yabu's instead assures only people from certain region and language can write there; I haven't seen in that sense a 'neighborhood-relay' one yet... But then I could guess there would be stable fixed relays by the numbers you mention along several ones that comes and goes as they see fit 🤔