The URI of a SOAP or REST Web service from which additional information about the BusinessEntity, Offering, PriceSpecification, or ProductOrService instance can be gained. The recommended domain is Offering, PriceSpecification, or ProductOrService. The recommended range is rdf:resource, i.e., the URI of a SOAP or REST Web service.
In principle, any existing or upcoming vocabulary for Web Services can be used in combination with GoodRelations, because the association between (a) the service description and (b) the GoodRelations description can be found via the Web Service URI value used with the gr:relatedWebService property.