A built-in, numerically controlled tailstock can be a valuable feature for automated processes. A fully programmable tailstock provides more rigidity and thermal stability. However, the tailstock casting adds weight to the machine.
There are two basic types of programmable tailstocks—servo-driven and hydraulic. Servo-driven tailstocks are convenient, but the weight they can hold may be limited. Typically, a hydraulic tailstock has a retractable quill with a 6-inch stroke. The quill also can be extended to support a heavy workpiece, and do so with more force than a servo-driven tailstock can apply.
Source: Buying a Lathe: Spindles and Tailstocks