Temperament wrote: If I did understood from the post I need to make an extra category and put products inside?
Yes. As said in the post, you can assign multiple categories to a product. Or only that one to discounted products, just as you like.
Temperament wrote: Will that duplicate products, one in "sale" category and one in regular category?
Not if you assign only the "discounted" category to these products. Either that, or you put the "discounted" category outside the "Product category" (where all regular categories are) and then assign both regular and "discounted" category to the products. Both ways work, it probably depends on what is more convenient for you, or makes better sense to you.
Temperament wrote: Can it not be done that Hika content module "takes out" only products from specific category that have Discounts on it?
Nope, there's no option for that. Instead you simply create a product listing module (or menu item, if you prefer) and set its "Associated category" to wherever your discounted products are. And this way it's much more flexible, too... at least in my humble opinion.