num
How many things are present in this zone without taking exclusive possession of it.
A plain Int, not a response and not a list. Not a response because a network has thousands of zones and almost none of them will ever see a population, so a statistic per zone would cost every model a great deal to report nothing; the statistics belong to whatever construct put the occupants there, of which there are a handful. Not a list because the zone does not need to know who is present -- whatever admitted them knows that -- and it does need the count to be free to read, since an admission policy asks "how crowded is that zone" and a claim asks "is anything in the way" on the hot path.
Zero for every zone in every model that does not use the population, which is all of them today: nothing outside this package can put anything in a zone.