Does the type of item you apply the Replicate Magic item infusion matter? I mean, if I infuse the replica of Bag of Holding in a dagger, does the dagger turn into a Bag of Holding, or it stays like a dagger with the same effects?
The artificer's Infuse Item feature states, under the "Infusing an Item" subheader (E:RftLW, p. 57):
Whenever you finish a long rest, you can touch a nonmagical object and imbue it with one of your artificer infusions, turning it into a magic item. An infusion works on only certain kinds of objects, as specified in the infusion's description. If the item requires attunement, you can attune yourself to it the instant you infuse the item.
The Replicate Magic Item infusion states (E:RftLW, p. 63):
Using this infusion, you replicate a particular magic item. You can learn this infusion multiple times; each time you do so, choose a magic item that you can make with it, picking from the Replicable Items tables below. A table's title tells you the level you must be in the class to choose an item from the table.
If yes, can the common armour variant magic items found in Xanathar's Guide to Everything, such as Cast-Off Armor, be created? Can I use this to get a set of Half Plate Armor from this infusion?
Best Answer
To infuse an item, you need the nonmagical version of that item.
The artificer's Infuse Item feature states (TCoE p. 12, E:RftLW p. 57, WGtE p. 179; emphasis mine):
The Artificer Infusions section at the end of the class description states (TCoE p. 20, E:RftLW p. 61, WGtE p. 181; emphasis mine):
And finally, the description of the Replicate Magic Item infusion states (TCoE p. 22, E:RftLW p. 63, WGtE p. 182; emphasis mine):
Each of these portions I've bolded make it clear that you need a nonmagical version of the item to infuse, whether you're using Replicate Magic Item or one of the other artificer infusions.
Even though the Dungeon Master's Guide doesn't actually say "[X] is the type of item needed to make [Y]" for any listed magic item (as far as I know), this sentence seems to refer to the fact that each magic item that can be replicated is essentially an enhanced version of a clearly identifiable mundane item.
For instance, boots of elvenkind are boots, so the clear implication is that you would need mundane boots to infuse if you want to use Replicable Magic Item to make boots of elvenkind. This follows the same logic as the Boots of the Winding Path infusion (TCoE p. 21, E:RftLW p. 62, WGtE p. 181), which does explicitly state:
In the case of your example, the description of cast-off armor states (XGtE, p. 136; emphasis mine):
Thus, you need a nonmagical version of whatever kind of armor you want to turn into cast-off armor. If you want cast-off half plate armor, you need a nonmagical set of half plate armor to imbue with that infusion.
The Replicate Magic Item infusion doesn't create a magic item from thin air - it gives you the ability to imbue a certain mundane version of that item with the necessary magic to function as that magic item.