Today the ad-hoc order body (all ad-hoc order items) is assigned order discounts that apply to everything in the order except those ad-hoc items that put that discount in the discount block list.
This is a negative option.
If you have many lines that specific line discounts that apply only to that line, you have to put each discount into the order and then block all but the one discount you want from all the other lines.
What would be a nice option to make this easier is a specific line item attribute that is a positive option for this order line item.
It can use the same sub attribute type as the order discount, but only apply to the line item it’s attached to. The name of this field can be anything you want. Perhaps specific_item_discount?