The Totals form lists the calculated purchase invoice sums and the sums you have entered on the different lines. You can therefore use the Totals form to compare these sums for the selected purchase invoice and check the invoice.
There can be various reasons for non-matching sums. An incorrect total sum may have been entered in the purchase invoice header, or the VAT sum may be incorrect. Also, the purchase invoice lines may contain incorrect sums or VAT codes, resulting in an incorrect calculated sum.