This option is related to taking the advance amount while opening the agreement.
When you are opening the agreement from the Rental module from left navigation panel you open a new agreement and fill in all details with the advance amount and when you press save the next screen is for allocating that advance amount.
Enter the payment methods and details if any and press save the advance amount receipt will be generated.
To see that invoice you have to go to the vouchers from left navigation panel and open receipts.
Here you will see the advance amount receipt generated and you can print it as well.