Input tax credit: HO may use ISD or invoice BOs; invoice value deemed open market value when recipient has ITC. Where an HO procures common input services for itself and BOs, it may either distribute input tax credit via the ISD mechanism (with mandatory ISD registration if used) or issue tax invoices to BOs; when a BO is eligible for full ITC the invoice value is deemed to be the open market value under rule 28, irrespective of whether specific cost components were included, and salary costs need not be mandatorily included in value where recipient BOs lack full ITC.