Allow me to share some details about adding class to your items or to a specific account.
If you're referring to adding Class to the Discount account in QuickBooks, you can do that by in the Journal Entry transaction. However, if you're referring to something else, please let me know in the comment section.
In case you want to track the sales by class with the discount, you can run the Sales by ClassDetail Report. Then, if the transaction has an applied discount, you can identify it under the Memo column. And check the Amount column to see how much is the discount being deducted. Here's how it looks like: