number - The bag id, where the item to use is located
slot
number - The slot in the bag, where the item to use is located
target
string? - unit the item should be used on. If omitted, defaults to "target" if a the item must target someone.
reagentBankAccessible
boolean? - This indicates, for cases where no target is given, if the item reagent bank is accessible (so bank frame is shown and switched to the reagent bank tab).
Details
Slots in the bags are listed from left to right, top to bottom. A 16 slot bag has the following slot numbers:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
A 6 slot bag has the following slot numbers:
1
2
3
4
5
6
This function is protected, though may still be called in the following situations:
When the (Guild/Reagent) Bank is open: Move the item to/from the (Guild/Reagent) Bank.
When the Merchant frame is open: Sell the item.
When the Mail frame is open and on the "Send Mail" tab: Add the item to the attachments.
When the item is equippable: Equip the item.
When the slot is empty: Do nothing.
When the item is a locked Lockbox: Show an error message that says it's locked.
When the item is a container (Clam Shell, Unlocked Lockbox, etc.): Open the Loot frame and show the contents.
When the item is a book: Open the book's Gossip frame for reading.
When the item is not a usable item: Do nothing.
When the item starts a quest: Open the quest Gossip frame for reading.
When the Trade frame is open: Add the item to the list of items to be traded. (I have not confirmed this)
When the Auctions tab is open: Add item for auction pricing and submission.
Patch 6.0.2 (2014-10-14): Protected. 
