Digital Servos are More Accurate and Repeatable in both THROW and CENTERING.
You can think of it This way: A digital servo turns right 3 degrees,(click, click, click). Now you bring it back to Center position,(click,click,click).
Each of these "Clicks"is a Precise and Equal movement of the servo. It is quite accurate.

An Analog servo is not as Precise, because its positioning is controlled by a Brushed Potentiometer (instead of a Digital circuit) and it is inherently
less accurate and Repeatable than a digital system.

Your particular Futaba Radio was designed Specifically to take advantage of this Digital Revolution. (I THINK you have to use Futaba digital servos though, in HRS mode). Resolution, Response, and Speed will be Enhanced while using HRS mode. It will also be more Expensive.

3 and 5 core servos have conventional motor construction. That Being wound copper wire in the Rotor/Stator.These are BRUSHED motors and are susceptable to wear and tear where the Brushes contact the Stators. More Cores are supposed to offer a Greater Degree of controllability.

Coreless means the Rotor is a Strong magnet. Its Motion is controlled by magnetic Fields, so there is no actual contacts (Brushes) to wear out.
These are the New Digital type of servo and Should have a longer service life.

Karbonite is Marketing Speak for "Strong Plastic Gear"...Lol.
Servos cannot be ENTIRELY metal, due to radio interference (Noise), that Glitches the signal. All servos have at least one plastic (Karbonite )gear for this reason.