Hello Shiva,

                  The other advances fees( Rs. 12,000 per semester) is for adjustments. Your expenses like BITS COOP, or canteens , etc. , if paid using your BITS ID(that will be issued to you once you get admission) are decducted from your total advance fees of Rs. 22,000/-(mess advance Rs. 10,000/- +other advances Rs. 12,000/-). So, its like the extra fees you pay beforehand, expenses from which will be deducted from your account and the remaining balance will be adjusted into the next semester's fees. 

Hope it helps!