Of the remainder, it is necessary to estimate the proportion of sales for which taxes are already collected, either because they are made to customers in states where the seller has nexus, or because the buyer pays use taxes, which is typical for most business-to business (B2B) sales. Once an estimate of un-taxed sales is developed, the overall sales figure
must be allocated across jurisdictions in order to apply the appropriate tax rates. Finally, in order to make going-forward projections of lost tax revenues, it is necessary forecast the key underlying variables for future periods. and local sales taxes.