Unused tax paid in a challan can be used in the remaining quarters of the current financial year and can also be carried forward to the next year. For Example, unused tax payment in Q1 of FY16-17 can be used for payment of TDS for Q2, Q3 and Q4 of FY 16-17 and if required also in FY 17-18.
Also, unused tax paid in a challan can be used for paying the TDS defaults of previous periods. For example, the unused amount paid in Q1 of FY 16-17 can be used for any old pending TDS payment.
Same Payment Challan can be used for TDS under different Sections/ Heads