In an AWS CloudFormation template, you define independent resources or resources that are implicitly dependent, in other words, reference other resources. For instance, you can define an EC2 instance and a security group. Then, you might choose to attach the security group to the instance or not. In these two cases, AWS CloudFormation organizes the order of the creation of resources differently.
In addition, you might have another resource such as an Amazon SNS topic in the same template which you might want to provision after your EC2 instance created. Maybe you have an explicit dependency between them.
In this blog post, I will talk about how AWS CloudFormation handles the order of creation for independent or implicitly dependent resources. I will also explain how you can add your own dependencies in a template, as well. Read more at The Order of Resource Creations on AWS CloudFormation post.