Explanation: When you add an AWS Config rule to your account, you can specify when you want AWS Config to run the rule; this is called a trigger. AWS Config evaluates your resource configurations against the rule when the trigger occurs. You choose which resources trigger the evaluation by defining the rule’s scope. The scope can include the following:
One or more resource types A combination of a resource type and a resource ID A combination of a tag key and value.
When any recorded resource is created, updated, or deleted AWS Config runs the evaluation when it detects a change to a
resource that matches the rule’s scope. You can use the scope to constrain which resources trigger evaluations. Otherwise, evaluations are triggered when any recorded resource changes.
Reference: http://docs.aws.amazon.com/config/latest/developerguide/evaluate-config-rules.html