The 'Order Discount % ' in the customer record is the default discount that will be copied to every new order started for this customer.
The 'Usual Prices ' in the customer record is the default price band/list/grade that will be copied to every new order started for this customer.
We say 'copied to' rather than 'applied to' because these settings are simply inherited onto new orders. They can then be changed to suit the order.
So imagine the following example.
Customer record for Mr Smith:
Usual Prices: NHS
Order Discount: 10%
From the Main Form choose Action New.
Select the customer (if not already selected on the form)
Notice the Pricing box shows 'NHS 10% discount'.
Notice if you add line item products to this order, the 'default' price band will be NHS (unless you change it).
You can put some line items onto the order with a different price band. (from a different price list eg. private).
If you find you are having to change the line item's price band too often on just this order, you could change the order's Prices box by double clicking it.
If you find you are having to change the usual price band, or order discount on many orders, change the customer's Usual Prices and/or Order Discount.
Notice at all times that if there is any order discount applied, it is displayed at the bottom of the main form next to TOTAL.