12-07 Capacity Reservations
EC2 instances are backed by a different kind of hardware, and so there is a finite amount of servers available within an Availability Zone per instance type or family.
Example
You go to launch a specific type of EC2 instance but AWS has run out of that server!
Capacity Reservation is a service of EC2 that allows you to request a reserve of EC2 instance type for a specific Region and AZ
The reserved capacity is charged at the selected instance type’s On-Demand rate whether an instance is running or not.
You can also use your regional reserved instances with your Capacity reservations to benefit from billing discounts.