...
The Customer Order field is populated automatically with the customer order if this purchase order is based on it. To populate the purchase order with the data from the customer order, click the Fill ( 
) button.
The Requisition field is available if the Enable Requisitions option is selected on the Administration > Settings > Purchasing and Warehouses form. The Requisition field is populated automatically with the requisition Requisition if this purchase order Purchase Order is based on it. To populate the purchase order Purchase Order with the data from the requisition Requisition, click the Fill () button.
The Basis field is populated automatically, if the purchase order Purchase Order is based on a document of the following types: Production Order or Quotation. You can manually select the basis document from the list (for this, click Show all from the drop-down list).
...
Notice the hyperlink below the Basis field – USD=3.65AED* WM-Service LimitedAED* MG China Import*VAT applied* Prices Include VAT– on the screenshot above (can be different in your document). The currency of the purchase order is determined according to the selected contract. To use other currency in the document, select another contract, or create a new one, if necessary. Click the hyperlink to change the price-related settings for the document. For details, see The Prices and Currency dialog box (Purchasing).
Use the Valid From and To fields if you want to restrict the interval during which this purchase order Purchase Order is valid.
In the Delivery Date field, you can specify the planned date of the inventory and services delivery.
...
At the bottom of the form, the Discount fields are intended to show the percent of discount and discount amount applied to the document items. The Total Due field shows the document total on the ordered goods and services and the VAT field shows the VAT total on the purchases.
...