Significance of Sales Tax and Use Tax on Purchase Order

There is one requirement pertaining to the Tax Calculation and it's impact on Purchase Order. We have our PR Creation, PO Creation ,PO Receiving Receipt creation happening in 3rd party external system . The 3rd party external system is integrated to Oracle Fusion ERP. Also the Tax calculation is happening in external system. Now our concern is if the Tax Calculation happens in 3rd Party External system, do we need to recalculate the Tax in Oracle ERP. Also we want to understand if Sales Tax and Use Tax are used on Purchase Orders? if yes, how they are applied on