A service model is the way that a firm offers intangible value to its customers. Different XaaS concepts describe broad category of service models, which offer customers product delivery and payment options that allow them to purchase access to products as a service. the most common ones are the three general cloud computing models: Software as a Service (SaaS), Platform as a Service (PaaS) and Infrastructure as a Service (IaaS).  See also https://en.wikipedia.org/wiki/Cloud_computing#Service_models.