A product or service model is a intangible entity that specifies some characteristics of a group of similar, usually mass-produced products, in the sense of a prototype. In case of mass-produced products, there exists a relation gr:hasMakeAndModel between the actual product or service (gr:Individual or gr:SomeItems) and the prototype (gr:ProductOrServiceModel). GoodRelations treats product or service models as "prototypes" instead of a completely separate kind of entities, because this allows using the same domain-specific properties (e.g. gr:weight) for describing makes and models and for describing actual products.
Examples: Ford T, Volkswagen Golf, Sony Ericsson W123 cell phone
Note: An actual product or service (gr:Individual) by default shares the features of its model (e.g. the weight). However, this requires non-standard reasoning. See http://wiki.goodrelations-vocabulary.org/Axioms for respective rule sets.
Compatibility with schema.org: This class is (approximately) a subclass of http://schema.org/Product.