EC2 RIs provide a discounted hourly rate and an optional capacity reservation for EC2 instances. AWS Billing automatically applies your RI’s discounted rate when attributes of EC2 instance usage match attributes of an active RI.
If an Availability Zone is specified, EC2 reserves capacity matching the attributes of the RI. The capacity reservation of an RI is automatically utilized by running instances matching these attributes.
You can also choose to forego the capacity reservation and purchase an RI that is scoped to a region. RIs that are scoped to a region automatically apply the RI’s discount to instance usage across AZs and instance sizes in a region, making it easier for you to take advantage of the RI’s discounted rate.