SERCOS / Digital vs External Analog Servos?

I'm relatively new to motion control and I am reading up on the various choices of Rockwell / AB hardware selection.

Everything I've seen so far has been integrated Kinetix drives over SERCOS. I've noticed that some instructions in the Motion Instruction Set are specific for external analog servo drives or drives on a CIP comms axis.

I am wondering how the Kinetix drives differ from external servos (say a servo over a analog interface (1756-M0XAE).

What do the integrated servos do that the external / analog servos can't (besides higher resolution).

well, with the integrated solution, you get an electric servo drive, and you can select a variety of motors (including linear). Logix knows all that it needs to know about the particular motor and hence the setup is relatively quick and painless.
THe analog servo can drive pretty much any device that takes an analog command. I've used it with valves, hydraulic motors, etc...
The structure (and some commands) will be different based on your selection. For example (MDO instruction sends a voltage to the output and as such is not available for an electrical motor)

Input resolution is a function of the feed back device.
Output resolution is usually 16 bits no matter what the format and is more than enough If you study controls a bit you soon find that 16 bits of output resolution is wasted unless the input resolution is much higher.