These devices are commonly used to extend the output abilities of microcontrollers so they can link with more Servos. Using a Serial Data Stream with a simple 2 wire protocol - you can usually control 6 additional Servos.

The SSC is normally based upon a small, mass produced MicroController which simply takes those signals, and stores the instructions - maintaining the Servos at the desired position.

They normally have a very small footprint - mostly dictated by the connectors. Most good robot component stores have at least one range.

These have mostly been replaced by I2c equivalents capable of controlling 16 servo motors, based on the PCA9685.