When you are limiting the updates and have regular 'turn' cycles and with smaller zones where you may need to use not a 3x3 window but 5x5 7x7 etc.. the further out you get the lower the update frequency. Objects that can directly interact need full updates, but further away where its mostly tracking can be much lower frequency. Of course significant events would be made to get thru (not discarded like incremental movements might be).

You would marshal your updates to be sub messages in larger packets to cut down the overhead (UDP you have direct control of this).

As mentione by others, a subscription system would be used to maintain lists of who gets updates for each zone (and now multiple lists if a high/low update frequency system is used).