In X3 we have different invoicing elements like Freight Charges, Discount %, Discount $ etc. that are used for applying the discount/charges on the orders/shipment lines (Refer below screen shot).
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Now if we are updating these freight elements from third party system via integration we use modes like web services and for this we need to know the line number/position of the freight element in the sales order to update right element with right value. For this we just have to create the Global Variable that holds the value of these freight elements.
Navigate through Development -> Data and parameters -> Development Setup -> Parameter definitions
Here you have to create the global variable (here GFRENUM) that holds the value of the line number of invoicing element in sales order or shipment screens.
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This Global variable can be used in coding to update Invoicing elements.
Hope this clarifies!