With the "normal" up function you can upp anything whereever, whenever and as often as you want, concerning that you have the money (Yang), the probably needed up-stuff and the item on the first place in your inventory.
As I already mentioned, you can up with the dt-up-function just one item once when you're at the right place for that (You must see any dt-blacksmith). The big advantage here is that you can up every item, no matter which blacksmith you see.