From the ontological perspective, a Product or Service Model is an intangible entity that specifies some characteristics of a group of similar, usually mass-produced Products. In case of mass-produced Products, there exists a relation hasMakeAndModel between the Products and Services Instance and the Product or Service Model.
Since eClassOWL and other products and services ontologies don't support this important disctinction, Product or Service Models are a subclass of Product or Service in GoodRelations. This allows using the same properties for (e.g. "hasWeight") for product models and actual products.
Examples: Ford T, Volkswagen Golf, Sony Ericsson W123 cell phone
Note: An Actual Product or Service Instance of which is known via a relation hasMakeAndModel that it is of a certain Product Or Service Model should by default share the features of this model (e.g. the weight). However, this requires non-standard reasoning. See the GoodRelations Wiki at http://www.ebusiness-unibw.org/wiki/GoodRelations for respective rule sets.