If you need to use elements other than the salary paid to calculate the withholding tax, you have to create items for this purpose only. (This could be the case if amounts paid abroad (not subject to tax) or if an amount paid could benefit from an abatement.
A statistical type of item type must be created:
You have to check "Item for the search of the withholding tax rate".
Take into account whether it is a periodic element or not:
If this item should be a deduction on another amount, it is possible to link the two items and to make a calculation:
Then you just have to enter the amounts in the calculation.