Discount is a rebate given to your customer on the value of items sold. It is way of encouraging bulk purchase or prompt payment.
Discount allowance in Quickbooks is an automated transactions - you only need to set up discount as an Item, set the percentage and click Ok to save changes.
When you want to record a discount allowed on invoice, follow these steps:
After adding all your items, rate and amount, click the ITEM drop-down menu and select discount to activate a deduction-based on your business policy.
Click save and close.
To add Discount as an item, follow my post on how to add new item in QuickBooks.
On this window, select Discount as item type and Enter Discount Allowed as item name.
To categorize your discount allowed, select Discount allowed as chart of account - follow my post on chart of account.