What is the difference between AWS_DEFAULT_REGION and AWS_REGION system variables?

They are not the same: one works in some situations and the other works in other situations.

e.g. I just learned that AWS_DEFAULT_REGION does not work with the AWS Java API library. See Force AWS library to obtain region from environment inside docker cluster

cf. 44151982, 36354423, Github aws/aws-sdk-go#2103