This video describes how to set 'Order to Upsell in Odoo15'. A sale order comes to Order to upsell only if one creates the invoice according to the ordered quantity and the delivered quantity is greater than the invoiced quantity.
If we go through particular product details, in the invoice tab of that product we can see an option called Invoicing policy. There we can see two options: ‘Ordered quantity’ and ‘Delivered quantity’. If we choose the Ordered quantity, one can create the invoice according to the ordered quantity of that product and if choose Delivered quantity the invoice will create according to the delivered quantity. To create an upselling invoice, the Invoicing policy should be ‘Ordered Quantity’.