The price you are filled at is your buy limit price. So the arca order is posted at 30.00 but the discretionary price is 29.90. Then if you execute you order for say 29.95 that is your fill. You wont get price improvement.
This type of order is like a hidden order. It is only advantageous for the person posting this type of order.
I can think of a few scenarios that this would be beneficial. If you want to penny the bid/offer and someone keeps posting their order ahead of yours. Say you want to buy at 30.00 and someone keeps getting in front of you at 30.01. Then you use this order. Set your display price at 30.00 and discretionary price at 30.02. That way you get filled at 30.02 ahead of the order at 30.01.