The rule 'Unused AWS Region' is designed to detect non-compliant AWS API calls made in forbidden regions, such as those logged by AWS CloudTrail. Non-compliant activities include actions taken in AWS regions that should not be used based on organizational policies, especially those that could lead to security vulnerabilities or data breaches due to lack of support or oversight. In terms of severity, this rule is marked as high due to the potential impact of unmonitored activity in unauthorized regions. It uses CloudTrail logs to identify specific API calls, checking the region to ensure compliance with allowed areas for operation. The rule requires monitoring for previously logged events that indicate unauthorized activity in regions that are deemed off-limits. The tests configured for this rule include scenarios checking for both authorized and unauthorized API calls in regions deemed acceptable or unacceptable, respectively. This rule is crucial for organizations to enforce their cloud policy compliance, especially in environments managing multiple AWS regions.