When selecting a site, if there is a rule available for that site, there will be a drop down menu and an Apply Rule button to apply the rule selected. This is shown to the right.
What any individual rule does depends on what the rule says. For more information about rules, see: Rule Management
Apply by Creating/Editing a Site
When you are creating or editing a site there will be an option to apply a rule to the site. The rules button will open a dropdown menu where you can select a rule. This will autofill any information in the rule which can help you save time.
The first picture shown is what you will see when creating a site.
The second picture shown is what you will see when editing a site.