It is collection of related resources. Compartments are a fundamental component of Oracle Cloud Infrastructure for organizing and isolating your cloud resources. You use them to clearly separate resources for the purposes of measuring usage and billing, access (through the use of IAM Service policies), and isolation (separating the resources for one project or business unit from another). A common approach is to create a compartment for each major part of your organization. For more information, see Overview of the IAM Service and also Setting Up Your Tenancy.
To place a resource in a compartment, simply specify the compartment ID in the "Create" request object when initially creating the resource. For example, to launch an instance into a particular compartment, specify that compartment's OCID in the LaunchInstance request. You can't move an existing resource from one compartment to another.
To use any of the API operations, you must be authorized in an IAM policy. If you're not authorized, talk to an administrator. If you're an administrator who needs to write policies to give users access, see Getting Started with Policies.

A usage report is a comma-separated value (CSV) file that can be used to get a detailed breakdown of resources in Oracle Cloud Infrastructure for audit or invoice reconciliation.
The usage report is automatically generated daily, and is stored in an Oracle-owned Object Storage bucket. It contains one row per each Oracle Cloud Infrastructure resource (such as instance, Object Storage bucket, VNIC) per hour along with consumption information, metadata, and tags. Usage reports generally contain 24 hours of usage data, although occasionally a usage report may contain late-arriving data that is older than 24 hours.
Usage reports are retained for one year.
Reference:
https://docs.cloud.oracle.com/en-us/iaas/Content/Billing/Concepts/billingoverview.htm
https://docs.cloud.oracle.com/en-us/iaas/Content/Billing/Concepts/usagereportsoverview.htm

A NAT gateway gives cloud resources without public IP addresses access to the internet without exposing those resources to incoming internet connections.

The Payment Card Industry Data Security Standard (PCI DSS) is a global set of security standard designed to encourage and enhance cardholder data security and promote the adoption of consistent data security measures around the technical and operational components related to cardholder data.
Oracle has successfully completed a Payment Card Industry Data Security Standard (PCI DSS) audit and received an Attestation of Compliance (AoC) covering several Oracle Cloud Infrastructure services and the Oracle RightNow Service Cloud Service. As a PCI Level 1 Service Provider, customers can now use these services for workloads that store, process or transmit cardholder data.
Reference:
https://www.oracle.com/cloud/cloud-infrastructure-compliance/