The document explains the 'order to upsell' feature within the Odoo v12 sales module, detailing the process from creating a sale order to invoicing and product delivery. A sale order appears in the 'order to upsell' menu when an invoice is created based on ordered quantity while delivered quantity exceeds the invoiced quantity. It provides a real-life example illustrating how to manage upselling through the invoice process within the Odoo system.