The URI of a SOAP or REST Web Service from which additional information about the gr:BusinessEntity, gr:Offering, gr:PriceSpecification, or gr:ProductOrService, or any other element, can be obtained. The recommended range is xsd:anyURI 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 this gr:relatedWebService property.