Vikalp - if a passenger's ticket is not confim in his desired train and if he has used Vikal option then he is liable to get a CNF berth (if Available) in another train of same route which departs after the first train.
In your case you have RAC ticket in 12721. Lets assume your ticket does not get confirm at the time of charting. As you hav opted for vikalp so you will get a CNF berth (if available) in another train that departs within 12 hours after departure of dakshin express.