Going through the internals of the template
The template is the main building block of CloudFormation. We can consider the template as a declarative instruction for CloudFormation—for example, what to create and how many.
The template file, whether in JSON or YAML format, consists of several elements, which we will describe in the rest of this section.
AWSTemplateFormatVersion
The AWSTemplateFormatVersion section is responsible for instructing CloudFormation as to what version of the template we are going to supply it with.
Not to be confused with the API version of CloudFormation, AWSTemplateFormatVersion is about the structure of the template.
When AWS has added this block to resolve potential breaking changes to CloudFormation, it can only have one value: "2010-09-09". This version is there from the very beginning of CloudFormation, but there is always a chance that AWS will add newer versions for template formatting. Until then, this section is optional...
