Using dynamic references to specify template values – AWS …

Dynamic references provide a compact, powerful way for you to specify external values that are stored and managed in other services, such as the Systems Manager Parameter Store, in your stack templates. When you use a dynamic reference, CloudFormation retrieves the value of the specified reference when necessary during stack and change set operations.

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/dynamic-references.html