When explicitly setting the region on an AWS Client, you should always prefer providing the value from the Enum Regions instead of a hardcoded String.
This will allow you to transparently support any change in the API and avoid mistakes.
This rule reports an issue when a hardcoded string is used instead of an available enum value.
Noncompliant code example
AmazonS3ClientBuilder.standard().withRegion("eu_west_1").build();
Compliant solution
AmazonS3ClientBuilder.standard().withRegion(Regions.EU_WEST_1).build();
