Rules allow you to define the content to which you want to apply feature behaviors applied. Rules are the IFs and features are the THENs of your Zone configurations. IF the request for the content matches this rule, THEN apply this feature. If the response includes that header, then apply that feature.
You create a rule by using one or more matches — questions about specific characteristics of the incoming requests or in some cases the responses from the origin. For example, is the request for this host? Does it contain this cookie or not? Is it for a JPG, TIF, or GIF? Does it need to have the right answer on all of these questions, or just one?
Creating and nesting rules
You can use a single rule match or you can combine matches to identify the content in the way you want, and the result is a rule. For example, if the request is for a JPG extension in /image/ folder path on host my.example.com, set a max-age of 1 day for it. The rule is comprised of matches on the host, path, and file extension. Based on the rule, set a caching feature behavior.
It is common to nest rules within rules to implement the behavior you want.
For Example, you set a rule that says, if the request is for this host and this path, set a feature to give all the objects a max-age of 12 hours. But then underneath this rule you set a nested rule that says that IF the file extension is CSS, PNG, JPG, or GIF, set a max-age of 1 week instead. Had you included the file extension in the parent rule instead of nesting it, you would not have been able to have two different max-ages for all objects and then the longer-lived file extensions.
Behaviors are the application of CDN features to your configuration, that is, how you want to handle and process the content passing through the CDN.