https://aws.amazon.com/ec2/autoscaling/faqs/

 AWS Auto Scaling lets you define dynamic scaling policies for multiple EC2 Auto Scaling groups or other resources using predefined scaling strategies.

 Referred to as a ‘fleet’. Fleet management refers to the functionality that automatically replaces unhealthy instances and maintains your fleet at the desired capacity.

EC2 Auto Scaling group (ASG) contains a collection of EC2 instances that share similar characteristics and are treated as a logical grouping for the purposes of fleet management and dynamic scaling.

EC2 Auto Scaling groups are regional constructs. They can span Availability Zones, but not AWS regions. Data is not automatically copied from existing instances to new instances

Step scaling policies and Simple scaling policies

Step scaling policies. When step adjustments are applied, and they increase or decrease the current capacity of your Auto Scaling group, the adjustments vary based on the size of the alarm breach.

Step scaling policies are a better choice than simple scaling policies, even if you have only a single scaling adjustment.

Simple scaling is that after a scaling activity is started, the policy must wait for the scaling activity or health check replacement to complete and the cooldown period to expire before responding to additional alarms. Cooldown periods help to prevent the initiation of additional scaling activities before the effects of previous activities are visible