In addition, it will be fun to see everyone's creations come together to make one big, populated region. You can also interact more directly with the cities adjacent to you by sharing power, water, commuters, and garbage services. Cities in the region will affect the demand for jobs, workers, and goods in other cities. In other words, to build the biggest city possible, you have to build several cities, which I found dull doing on my own. For example, you need something like 30,000 middle-class commercial workers before you can have a middle-class commercial skyscraper. In fact, before your city can have the biggest and best skyscrapers, you must meet hidden quotas for workers for the entire region, which are basically impossible to reach with just one city. In Sim City 4, your city is part of a region, which can contain many cities, all player-made.
It's an ad-hoc multiplayer that's effectively play-by-mail, only it mails every time you save. It's possible to sync all of these files by Dropbox while playing simultaneously. Quite simply, an in-game region of cities is composed of individual city files. I got the idea from an old post on sc4devotion forums, which is AFAIK the only time it's really been done. This a multiplayer region for us to play some Simcity 4.