Each sector is colour coordinated. Sectors are broken up inside of a building by doors - if you have multiple rooms that flow into each other without a door, they will all be considered one sector. Jail doors do not seem to count towards this, but large jail doors and normal doors do.
= = History ==
Since [[Alpha 2]], the primary use of sectors is to assign guards in the [[Deployment]] window.
Since [[Alpha 6]] you're now able to dedicate cell blocks to prisoners of maximum security (high risk), minimum security (low risk) or the general prisoner population.