Those are usually very busy sectors with a lot of office buildings and towers. You'll also find most shops here, as well as hospitals, banks and restaurants. Houses tend to be tiny and there's no factory there. As those districts are located in the center of the map, you will find plenty of people there.
This area can houses most of the building types. However, this a poor area, and even if all kind of shops are available here, you'll mainly find small houses and most factories.
Those districts usually have large mansions, parks, practical shops and services. You won't find a weapon shop here. And the only kind of factory allowed is the food related one. This is a rich area, with usually, larger buildings. And of course, it's less populated than the two previous kind of district.
This is a special district. There's only one of them per map. This district is the Military Faction's home. You can move across it freely. Howevern entering any building will cause the soldiers to attack you on sight, as those buildings are restricted areas. However, who knows what you may find by exploring the location.