The bank filler is an item found exclusively within a player's bank, and cannot be withdrawn. It fills the entire player's bank, with the amount filled dependent on the number of bank spaces remaining.
The bank can be filled with bank fillers by enabling them in the bank settings menu on the top right corner of the interface, just below the X. Players can be remove the desired amount of bank fillers from their bank by right-clicking a bank filler.
Having a bank full of filler items allows players to decide which items they want deposited when using the Deposit inventory or Deposit worn items buttons. For example, if players are crafting leather items, they can remove the needle and thread from their bank, enable the bank fillers, and be able to use the Deposit inventory button without having to consistently withdraw the needle and thread.