Just like AWS resources, CloudFormation stacks can also have return values, called outputs. These values are entirely user defined. If you don't specify any outputs, then nothing is returned when your stack is completed.

Outputs can come in handy when you are using a CI/CD tool to create your CloudFormation stacks. For example, you might like to output the public hostname of an ELB so your CI/CD tool can turn it into a clickable link within the job output.

You'll also use them when your are linking together pieces of your layer cake. You may want to reference an S3 bucket or security group created in another stack. This is much easier to do with the new cross-stack references feature, which we'll discuss later in this chapter. You can expect to see the Ref and Fn::GetAtt functions a lot in the output section of any CloudFormation template.