This is a memory latch that can reliably store one digit. When you store 1, the light is on. If you store 0, the light stays off. This may be confusing for some, but play around to see how you can store a digit.
( HINT: the set memory button will allow you to store a new number when it is on 1. When the set memory button is set as 0, you cannot make any modification to the stored memory. Use the ´stored memory´ button to toggle the digit you are storing)