Payment Discount Illustration is usually used when you make sales or buy items and it's due for payment. So you can receive or give discounts when paying invoices earlier than the due date.
Here are the steps to do Payment Discount Illustration:
I. Payment Discount Illustration Setup Through Debt Menu
Step 1. Select more menu - 'Payment Discount Illustration Setup'

Step 2. Select on add illustration

Step 3. Fill the discount illustration detail

II. Payment Discount Illustration Through Debt Menu
Step 1. Select more menu - Payment Discount Illustration

Step 2. Select filter and fill with your customer that has debt

Step 3. Debt payment discount illustration

You might also like to read these articles below:
You can also Setup Debt Limit and Maximum Overdue for Customer
In case you want to pay bulk invoices you can check AR & AP Collection
You could pay an invoice by Add Payment to Credit Sales / Purchase