As a service
Internally these often complex systems generally possess a high degree of internal automation which generally provide varying levels of fault tolerance and resiliency, the ability to scale up/down in or out to meet capacity and performance requirements of the workloads submitted to the service by its users/consumers, and are usually intended to operate their day to day functions without the need for human intervention. IaaS (Infrastructure as a service) features most commonly included in this automation package are compute, storage, network, telemetry, and logging/accountability features, but most IaaS components owe some portion of their  workload to these services. The current cloud computing ecosystem contains multiple cloud providers, each with their own menu of these services for their customers to consume on-demand or in some cases even with pre-scoped capacity agreements. Because (X)aaS services are generally based on open source projects with little to no licensing costs, ideally require little to no human intervention to perform their tasks/roles, could be scaled out on demand to the full physical resource capacity of an entire datacenter region, are maintained and supported internally by the cloud provider, and use resources that are expendable for reuse  by other IaaS tasks and services, XaaS services represent a large cost savings over traditional vendor-provided infrastructure and server based equivalencies, which often include vendor-specific lock-in features and imposed product feature set limitations which focus more around supporting that vendor's product portfolio and feature roadmap than similar open source projects.