When using the layered cake approach, it's very common to want to use outputs from one stack as inputs in another stack. For example, you may create a VPC in one stack and require its VPC ID when creating resources in another.

For a long time, you needed to provide some glue around stack creation in order to pass the output between stacks. Cross-stack references provide a more native way of doing this.

You can now export one or more outputs from your stack. This makes those outputs available to other stacks. Note that the name of this value needs to be unique, so it's probably a good idea to include the CloudFormation stack name in the name you're exporting to achieve this.

Once a value has been exported, it becomes available to be imported in another stack using the Fn::ImportValue function – very handy!

Make sure, however, that during the time an exported value is being referenced, you are not able to delete or modify it. Additionally, you won't be able to delete the stack containing the exported value. Once something is referencing an exported value, it's there to stay until there are no stacks referencing it at all.