Amazon EC2 Fleet is a new feature that simplifies the provisioning of Amazon EC2 capacity across different Amazon EC2 instance types, Availability Zones and across On-Demand, Amazon EC2 Reserved Instances (RI) and Amazon EC2 Spot purchase options. With a single API call, now you can provision capacity across EC2 instance types and across purchase options to achieve desired scale, performance and cost.
You can create an EC2 fleet specification defining target capacity, which EC2 instance types work for you, and how much of your fleet should be filled using On-Demand, RI and Spot purchase options. You can also indicate whether EC2 Fleet should take into account the number of cores and amount of memory on each instance or consider all instances equal when scaling. EC2 Fleet then launches the lowest price combination of instances to meet the target capacity based on these preferences.
Amazon EC2 Fleet is now available in the AWS GovCloud (US-West) Region. Visit AWS GovCloud (US) homepage to learn more about these regions. To learn more about Amazon EC2 Spot Instances, visit our documentation.
from Recent Announcements https://aws.amazon.com/about-aws/whats-new/2019/06/amazon-ec2-fleet-now-available-in-the-aws-govcloud-us-west-region/