You have probably heard of many other X-as-a-Service offerings such as Storage-as-a-Service, Desktop-as-a-Service, Network-as-a-Service, Backend-as-a-Service, Function-as-a-Service. These other models are merely subsets or aggregations of SaaS, IaaS, or PaaS. Categorizing them into the three standard models simplifies any cloud conversation you may have.