`AWS_REGION` vs `REGION` environment variables

In `.ebextensions/options.config` it is the `AWS_REGION` environment variable that is set, but in `app.js`, the code refers to the `REGION` environment variable to set `AWS.config.region`. Inspection of running system on launched EB environment shows that indeed `REGION` is actually not set.

Although the code still appears to work, I believe that the intention was to say `AWS.config.region = process.env.AWS_REGION` instead (otherwise `AWS_REGION` is not used anywhere). I'm a bit puzzled as to why the code still works when `AWS.config.region` is set to undefined (especially after one switches to use `nodejs-tutorial` for table name). How does the running process know to use the correct region in this case?