You can define the basis of tax calculation in IFS/Purchase for a given purchase order.
The Use Price Incl Tax option in the Purchase Order/Order Details tab determines the tax calculation basis for the purchase order part line, no part line and the charge lines. The default setting for this option is retrieved from the supplier defined in the header. If not, it is retrieved from the Company/Supply Chain Information/Procurement tab. If the purchase order is created from an order quotation, then the setting for this option is retrieved from the order quotation approval. If the purchase order is created from a purchase requisition, then the setting for this option is retrieved from the requisition line. It is possible to change this value before saving the record. When the purchase order is created by using the duplicate or copy object options, the default value is retrieved from the purchase order.
When the purchase order is created directly from a purchase requisition, then the default value of the option is retrieved from the corresponding purchase requisition line and the option is non editable. When the purchase order is created from an order quotation, then the default value for the option is retrieved from the particular order quotation approval. When the purchase order is created from a shop order, work order or project, the option is always not enabled by default.
When creating a purchase order part line, no part line or charge line for a particular purchase order header, the price or price including tax, and the tax is calculated based on whether or not the Use Price Incl Tax option is enabled.