The Invoice Basis Exception only appears if you have configured the settingCreate supplier invoice basis during Arrival Reporting?.
If you have activate the setting regarding creating supplier invoice bases during arrival reporting, you can make exceptions from this setting for the supplier. If this exception is activated, no invoice basis will be created when purchase orders for that supplier becomes arrival reported. The purpose is to be able to except the suppliers where the received invoices are not specified per purchase order or in another way will not generate a good invoice basis. You then risk creating invoice bases that never will be linked to a supplier invoice, which in its turn will result in that the Invoice Basis list will not show correct information about "Arrived, not linked to invoice" as well as incorrect purchase statistics. It can in these cases be better not to create invoice bases for the supplier in question and instead update the purchase statistics at the time of the arrival. You will find a corresponding settings for purchase order on subcontract in the Update Work Center procedure.