ALB Least Outstanding Request routing

Hello.

I checked the following document, but it did not mention the algorithm for determining the target.
I think that the details of the function itself are managed by AWS, so there is no description of how the target is determined.
https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-target-groups.html#modify-routing-algorithm

This is just my guess, but I think they are tracking requests in some way.

As per my understanding, ALB knows how many BackEnd registered with it, and when it sends traffic to BE , it keeps count of number of requests in progress to each BE, and determine which one has least inflight requests and choose that for next request.