What exactly are the differences between settlements and districts in the kingdom building rules as presented in Ultimate Campaign?
My DM believes that there is essentially no difference; each district has its own base value, etc. I can't find any sort of reference to support or deny this, but it seems that there isn't much actually quantifying what the two actually are.
A settlement is a collection of districts. In particular, "A settlement's population is approximately equal to the number of completed lots within its districts × 250. a grid that has all 36 lots filled with buildings has a population of approximately 9,000."
Note both that districts in the sentence above is plural, and also that this puts an upper bound on the population of a single district of about 9,000. Obviously, a metropolis of 360,000 people wouldn't fit into a single district, and in fact that would be expected to occupy about 40 "districts"
This is important because, of course, if you treat settlements and districts as equivalent, you'll never get any settlements larger than a small city.
Oh, further to prior:
"Most settlements only have 1 district. If your District Grid is full and you want to add another district (for example, if you run out of available lots in that settlement and want to construct additional buildings), you can create an additional district for that settlement by paying the preparation cost for the settlement's terrain as listed on Table: Terrain and Terrain Improvements."
So while most settlements only have one district, there are official rules for multi-district settlements. They're just not very common, because large cities and metropoli (metropolises) aren't very common.
Nothing really for me to add: OQ and Ascalaphus are absolutely right.
Settlements also possess all the settlement modifiers (like lore and such) from all their districts, whereas districts do not concern themselves with those statistics.
Random events can target settlements, but they won't target districts.
I'm essentially repeating what Orfamay said: It doesn't matter until you get multi-district Settlements.
Thanks, all. I got my DM to look over it again, and he agreed that he misread things the first time through.