Input Tax Credit apportionment guides ITC restriction and attribution for mixed, taxable and exempt supplies with banking option. Apportionment under Section 17 restricts ITC where inputs are used partly for business and partly for other purposes or for both taxable and exempt supplies; credit must be attributed to taxable use. Section 17(3) defines value of exempt supply to include nil-rated, wholly exempt, non-taxable supplies, reverse-charge supplies, specified securities transactions and certain land and building sales. Banking and similar financial entities may opt for proportionate availment or a monthly fifty-percent claim with lapse of the remainder. Section 17(5) lists blocked credits; Rules 42 and 43 set attribution and reversal mechanics. A circular exempts certain e-commerce operator ITC reversals under section 9(5).