First, you create a single customer to use as a template and set it up with default values as needed. Then specify this customer account in Accounts receivable parameters -> General tab. Give this customer a special account number, i.e. OneTIme Customer, and set the customer as Stopped at All to prevent its accidental usage.
Then, when creating a new Sales order, check the one-time customer box, and a new customer will be created at the same time that the sales order is created. There is a separate number sequence for one-time customers which can be different from manually added customers for visibility. The one-time customer box on the customer record is for reference only once created. There is nothing special about one-time customers except for how they are created.
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