nopCommerce 2.3 discount logic

Scenario:
I would like to provide some discount for the given order. Go to Order->Products, click the "Edit" button to enter the discount as a discount for one of the purchased products (see no other way, although in the given situation this is the discount for the whole order and I would prefer to have the according option). When the Edit button is clicked I get possibility to enter discount "Incl tax" or "Excl tax". If I use the first option - I cannot see the entered discount when I click the "Save" button and return to the product grid. If I use the second option - I can see the discount in the product grid, still it does not seem to produce any other effect. At least the PDF invoice is still the same and does not mention the discount.

For my eyes it would be more logical if the following would be the case:
1) Entered discount is visible in both cases ("Incl tax" and "Excl tax")
2) Entered discount  is present in the order information (PDF invoice, for instance)

It would also be nice to have an option for providing order level discount.

The question: do I understand the situation correctly and it is probably somewhat strange or I simply try to achieve the desired result it in the wrong way?

Categories also have a Discount Tab

Also, discount can be set up for order in Promotions > Discounts

Thank you,
The purpose was to provide a "non-regular" discount for an already placed order.  It turned out that it was almost possible: I could enter the product discount, though it produced no visible effect, causing a question: is that a feature, a bug, or a rudiment of a feature that has not really been implemented.