The Controller Slave is a block added by Storage Drawers. It is used to automatically insert items into a system from another entry point, without the need to insert directly into Drawer Controller, which there can be only one existing in the system. It does not have an inventory or interface. The Controller Slave must be connected to the drawer system within 12 blocks of the Drawer Controller, but can easily be hidden behind a wall of drawers.
Items are inserted into the drawer system as if they have been put into the Drawer Controller, so the empty drawers closest to the Drawer Controller will be used first, for items that are not already in the system.
It is often mistakenly believed that the Controller Slave enables the player to have multiple walls of drawers attached to one Drawer Controller. A Wood Trim or Framed Trim is used for this purpose. A Controller Slave simply exposes additional faces for its controller block for item flow automation.