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This is an optional helper that eases the AWS region setting. This helper is supported just for the query APIs. The RESTful S3 API binds the buckets to specific regions, therefore this call does not apply. Notably, the SES API does not have region support, while the default entry point is 'us-east-1'. It can't be changed via the client.setRegion() method as this helper is specifically disabled for this API till Amazon enables SES for other regions. The IAM client doesn't support regions as the service has just a single publicly accessible end point: iam.amazonaws.com.
The region actually specifies the AWS Endpoints. aws2js constructs internally the endpoint by using this methodology: apiPrefix + regionName + '.amazonaws.com'. The apiPrefix is hard coded into the default client configuration which is applied when you call the load() method. The regionName is the argument of this method which must be one of the region names as described into the AWS Endpoints documentation.
The regionName is restricted to one of the values: us-east-1, us-west-1, eu-west-1, ap-southeast-1, ap-northeast-1.
The us-gov-west-1 restricted region for EC2 / CloudWatch or the us-gov restricted region for the IAM client are unsupported. Most probably they won't be supported in the future as I doubt any US Gov agency would actually need my library for doing AWS work with node.js.