Input-tax credit allocation: HO may use ISD registration or invoicing to enable BOs to claim ITC, subject to valuation rules. For services between distinct persons, HO may either distribute ITC through the ISD mechanism (requiring ISD registration) or issue tax invoices under section 31 so BOs can avail ITC; ISD distribution is allowed only if services are attributable to or actually provided to BOs. Invoice value for internally generated supplies is governed by rule 28 read with section 15(4): if the recipient BO is eligible for full ITC, the invoice value is deemed open market value irrespective of included cost components, and HO need not include employee salary costs in taxable value where recipient is not eligible for full ITC.