In general usage, product may refer to a single item
In general usage, product may refer to a single item or unit, a group of equivalent products, a grouping of goods or services, or an industrial classification for the goods or services, such as a single carton of brand X milk, a single customized interior design, a single piece of lumber, or a single hour of technical support. Serial numbers are used to identify certain unique products. A vehicle identification number identifies a unique motor vehicle.
And what about “defective products”? Usually it’s a single instance or a few instances of unique products not meeting specifications or standards. Every product is unique in the sense that it cannot be sold to different customers such as customer A and customer B at the same time, or sold twice at the same time to a single customer. An invoice is a business document requesting payment for actual product delivered. Double billing is the error of charging a customer twice for the same unique product.