Unless global formulary apportionment includes every member of an MNE group, it must retain a separate entity rule for the interface between that part of the group subject to global formulary apportionment and the rest of the MNE group. Global formulary apportionment could not be used to value the transactions between the global formulary apportionment group and the rest of the MNE group. Thus, a clear disadvantage with global formulary apportionment is that it does not provide a complete solution to the allocation of profits of an MNE group unless global formulary apportionment is applied on the basis of the whole MNE group. This exercise would be a serious undertaking for a single tax administration given the size and scale of operations of major MNE groups and the information that would be required. The MNE group would also be required, in any event, to maintain separate accounting for corporations that are not members of the MNE group for global formulary apportionment tax purposes but that are still associated enterprises of one or more members of the MNE group. In fact, many domestic commercial and accountancy rules would still require the use of arm’s length prices (e.g. customs rules), so that irrespective of the tax provisions a taxpayer would have to book properly every transaction at arm’s length prices.