Mission expansion

A mission expansion is a random chance (there's percentages set in the code somewhere) that your mission will change into the expanded missions, upon unit arrival. Like a small protest can grow into a medium, a domestic can turn into an officer down, or flood size can go anywhere from small to giant. Basically it just changes the call on you when the first unit shows up. Most times it makes it bigger and requires more units, but in some cases, the unit requirements can change, making some units on scene no longer necessary.

I believe that is correct. Disabling it under settings will stop calls from growing. It's helpful particularly if you find yourself building calls up in the red that you can't answer. I leave it on because it helps the pot grow faster.