why qty deducted is negative?

- I have seen before in an application a qty returned in purchase or sales returns or any transaction which deduct from the inventory (inventory out), it shows the quantity in minus like this (-20) i.e , even the discounts on any order or invoice it shows it the same way.

- Is that the way it should work in applications? or some does it and some does not?

- Is there a benefit from it?