Why the discount is not relevant to invoice class?

Let me provide some insight regarding the discount that you may find helpful.

If you want to apply the discount to a certain class, we'll need to construct a service item for it. Then, manually add it to the sales invoice so that you can apply it to a certain class. The following stages will lead you through the process of creating a discount service item: 

1. Click the Gear icon at the upper right corner and choose Products and Services.
   
2. Tap the New button.
   
3. Select Service.
4. Name the item, for example, "Discount."
5. Enter all the necessary details.
6. From the Income account drop-down menu, choose the account for discounts.
7. Hit Save and close.
    

Here's how to include it on your invoice:

1. Click the + New Plus icon on the left panel and choose Invoice.
2. From the Select a customer drop-down menu, pick the correct customer.
3. Enter the Invoice details.
4. Fill in the necessary details in the Product/Service, Description, Qty, Rate, Amount, and Class fields. I've attached a screenshot below for your reference.
   
5. Press Save and close.

To know more about the discount feature in QuickBooks, check this out: Add a discount to an invoice or sales receipt in QuickBooks Online. It includes instructions on how to apply a specific amount to your invoice or sales receipt.

In addition, you can read the following article to learn how to use your sales form templates in QuickBooks Online, as well as what other information you can include: Customise invoices, estimates, and sales receipts in QuickBooks Online.

You can always come back to the Community if anything may arise while working in QuickBooks Online or if you have any follow-up questions about sales discounts. I'd be glad to help.