Answer 1

In Elastic Load Balancing, when an Application Load Balancer handles a request, the trace information is added to the X-Amzn-Trace-Id header

You can log this unique identifier and then use it to troubleshoot issues with your load balancer. For example, you can use the X-Amzn-Trace-Id header to identify when many similar requests are received from the same client within a short time. If you have many layers in your stack, you can also use the X-Amzn-Trace-Id header to track a unique request across all the layers.

Source :
1. https://aws.amazon.com/premiumsupport/knowledge-center/trace-elb-x-amzn-trace-id/

2. https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-request-tracing.html