Refill keys are used to change the settings on all modern coin operated machines. The keys are used to access the following on most modern fruit machines: - 
 
Meter readings (Cash In/Out Notes In/Out)

Volume Control (View & Change The Sound level)

Percentage Payout (View aiming & the actual payout percentage)

Float Level (View the hopper level without empting the machine)

Refill (Refill the machine without the need to open the machine)

Hopper Top-Up (Used to confirm the machine has a full float

View Win Information (View Last win, Last Bank  etc)
These keys can also be used for various other settings on other coin operated machines 
 
87000 Standard Fruit Machine Refill Key