Explanation
Infrastructure as a service (IaaS) is a type of cloud service model in which computing resources are hosted in the cloud. Businesses can use the IaaS model to shift some or all of their use of on-premises or colocated data center infrastructure to the cloud, where it is owned and managed by a cloud provider. These infrastructure elements can include compute, network, and storage hardware as well as other components and software.
How Does IaaS Work?
In a typical IaaS model, a business-which can be of any size-consumes services like compute, storage, and databases from a cloud provider. The cloud provider offers those services by hosting hardware and software in the cloud. The business will no longer need to purchase and manage its own equipment, or space to host the equipment, and the cost will shift to a pay-as-you-go model. When the business needs less, it pays for less.
And when it grows, it can provision additional computing resources and other technologies in minutes.

Budgets are set on cost-tracking tags or on compartments (including the root compartment) to track all spending in that cost-tracking tag or for that compartment and its children.
https://docs.cloud.oracle.com/en-us/iaas/Content/Billing/Concepts/budgetsoverview.htm