I really confused and didn't know what is the difference between "fleet" and "cluster" in aws. For me, they are all "a bunch of something", and can't distinguish them.

I found that nobody would ask this question since they are on a different level and it's a definition problem that I misunderstand the critical part of them.

We call it an EC2 fleet.
We call it an ECS cluster.
We call it an EKS cluster.