Input Tax Credit apportionment: formulaic allocation and mandatory reversal for exempt and non business use under Rule 42. Rule 42 mandates calculating C1 = T - (T1+T2+T3), then C2 = C1 - T4. For each tax period compute D1 = (E/F) x C2 and D2 = 5% of C2. Eligible credit C3 = C2 - (D1 + D2). The aggregate of D1 and D2 must be reversed and declared separately for each tax head in FORM GSTR 3B or FORM GST DRC 03. Final annual or project wise computations compare provisional and final D1/D2 (and C3 for commercial portions), with excesses reversed or claimed before the September return deadline and interest payable on delayed reversals.