A ProductGroup represents a group of [[Product]]s that vary only in certain well-described ways, such as by [[size]], [[color]], [[material]] etc. While a ProductGroup itself is not directly offered for sale, the various varying products that it represents can be. The ProductGroup serves as a prototype or template, standing in for all of the products who have an [[isVariantOf]] relationship to it. As such, properties (including additional types) can be applied to the ProductGroup to represent characteristics shared by each of the (possibly very many) variants. Properties that reference a ProductGroup are not included in this mechanism; neither are the following specific properties [[variesBy]], [[hasVariant]], [[url]].