Health checks for your target groups – Elastic Load Balancing

Health check reason codes. If the status of a target is any value other than Healthy, the API returns a reason code and a description of the issue, and the console displays the same description in a tooltip.Reason codes that begin with Elb originate on the load balancer side and reason codes that begin with Target originate on the target side.

https://docs.aws.amazon.com/elasticloadbalancing/latest/application/target-group-health-checks.html