This button helps swap or change cashiers on a single system. So when one cashier is working on a system, and another cashier somehow needs to work on the same system, all he/she has to do is use this functionality. When you click on this button, below screen will open.
As you shown above, the window will give information that who is currently logging into a system. With the newly fed cashier ID, the system will switch to the transactions made by the related cashier and any further transaction made, would be recorded in his/her records with cashier whose is newly.