The Door Servo is a utility controlled by a Door Control, Door Timer or other circuit that opens doors for staff and prisoners. It requires Remote Access to be unlocked. In order for the door servo to work, it must be placed with the motor (bottom, grey circle) over the top of the door and the servo itself (square box with a light on it) out of it.
The servo can be connected to any type of door (including road gates), and the light on the top turns green when activated. Chaining multiple servos together will cause them to all open simultaneously whenever a signal to open any one of them is received.