Input tax credit distribution: HO may use ISD or invoice BOs, invoice value deemed open market value when recipient has full credit. For services between distinct offices, the HO may either distribute ITC via the ISD mechanism (with mandatory ISD registration if used) or issue tax invoices to BOs so BOs can claim ITC; where a recipient BO is eligible for full ITC, the invoice value is deemed the open market value under rule 28 irrespective of included cost components, and a nil invoice may be so deemed; where the BO is not eligible for full ITC, HO employee salary need not be mandatorily included in taxable value.