In Ubercart, the product model (usually defined by a manufacturer) or SKU (Stock Keeping Unit) is the main identifier for a product across the various systems. When creating your products, deciding on a standard for setting this field up front will save you a lot of grief later on. For the purpose of this handbook page, we will use the term "SKU" to refer to the field, though in some parts of Ubercart it may still be referred to as "Model."
It is important to know that while every product node must have at least one SKU defined on the product form, you may end up using more than one SKU per product node. There is a SKU adjustment system available through the Attribute module that may be used to alter the product SKU depending on a customer's attribute selections. Once you have attached attributes and options to a product, go to the Adjustments tab on the product edit page to alter the SKU when a particular combination of attribute options are selected by a customer. When the product gets added to their cart, it will contain the adjusted SKU. It will be this SKU which may be checked for any number of things including stock, file downloads, recurring fees, etc.
Product features are generally attached to a product on a per-SKU or all-SKU basis. You can read more about product features (like file downloads) in their individual sections of the handbook. Suffice it to say that using the attribute system with adjusted SKUs is the way to sell a product as a physical shipment or digital download from one product page or sell a different length role promotion for site membership from one product page. Adjustments may also just be used internally for order fulfillment, as the shipper can more easily locate different products using unique SKUs.