Explanation/Reference: https://docs.cloud.oracle.com/en-us/iaas/pdf/gsg/OCI_Getting_Started.pdf

Explanation
You can filter Costs Analysis Tools by following three ways
To filter costs by dates
To filter costs by tags
To filter costs by compartments

Explanation
Oracle Cloud Infrastructure is hosted in regions and availability domains. A region is a localized geographic area, and an availability domain is one or more data centers located within a region. A region is composed of one or more availability domains. Most Oracle Cloud Infrastructure resources are either region-specific, such as a virtual cloud network, or availability domain-specific, such as a compute instance. Traffic between availability domains and between regions is encrypted. Availability domains are isolated from each other, fault tolerant, and very unlikely to fail simultaneously. Because availability domains do not share infrastructure such as power or cooling, or the internal availability domain network, a failure at one availability domain within a region is unlikely to impact the availability of the others within the same region.
The availability domains within the same region are connected to each other by a low latency, high bandwidth network, which makes it possible for you to provide high-availability connectivity to the internet and on-premises, and to build replicated systems in multiple availability domains for both high-availability and disaster recovery.
Oracle is adding multiple cloud regions around the world to provide local access to cloud resources for our customers. To accomplish this quickly, we've chosen to launch regions in new geographies with one availability domain.
As regions require expansion, we have the option to add capacity to existing availability domains, to add additional availability domains to an existing region, or to build a new region. The expansion approach in a particular scenario is based on customer requirements as well as considerations of regional demand patterns and resource availability.
For any region with one availability domain, a second availability domain or region in the same country or geo-political area will be made available within a year to enable further options for disaster recovery that support customer requirements for data residency where they exist.

Explanation
When creating a compartment, you must provide a name for it (maximum 100 characters, including letters, numbers, periods, hyphens, and underscores) that is unique within its parent compartment. You must also provide a description, which is a non-unique, changeable description for the compartment, from 1 through 400 characters. Oracle will also assign the compartment a unique ID called an Oracle Cloud ID.
You can create subcompartments in compartments to create hierarchies that are six levels deep.