Starting today, you can use EC2 Fleet with targeted On-Demand Capacity Reservations. On-Demand Capacity Reservations enable you to reserve compute capacity for your Amazon EC2 instances in a specific Availability Zone for any duration. For targeted Capacity Reservations, instances must specifically target the Capacity Reservation to run in the reserved capacity. Until now, there was no option to use targeted Capacity Reservations when launching an EC2 Fleet.
Source:: Amazon AWS