The basic fare is what the class and distance tables give for the journey, and is the only part of the fare that concessions discount and that the Tatkal percentage is applied to. The reservation fee, the superfast charge and GST are added on top and charged in full regardless of any concession.
That is why a 50% concession never halves the ticket, and why a Tatkal charge shown as 30% is 30% of a smaller number than the fare you see.
Also written or searched as: base fare · basic fare meaning · fare components · fare breakup