One explanation I found online - "When railway ticket is booked you must have noticed your booking status as CNF/D1/100/NC. CNF means confirmed and NC means ‘No Choice’. In case you have selected seat preference like window seat etc. and that seat is not available then it is indicated as NC in ticket and another available seat is allotted." click here
Another short explanation - "NC Means….No Concession….Confirmed by IRCTC customer care" click here
Frankly, neither seems very logical since if berth number is given, you already know whether your choice was given or not. And similarly what is the point of mentioning No Concession, I dont understand.