Usually different codes stand for type of use the motor is intended for.
For example, for the same engine model:
C would be Pleasure, Continuous Use 100hp rated motor (this motor would see 50-200hrs per year)
D would be Commercial, Continuous Use 80hp rated motor (this motor would see 200-1000hrs per year)
C/HD would be Heavy Duty, Continuous Use 65hp rated motor (this motor would see 2000-3000hrs per year) - good example is GenSet (generator) motor
Heavier the duty, and more expected hours, with prolonged continuous use, the less HP factory will rate the motor in order to ensure longevity.
The above are just examples, I dont have specific experience with MD11 series engines