CZLB is disabled by default on NLB and GLB and is enabled by default on the ALB. Essentially the function of CZLB is how your load balancer distributes traffic among AZs. If you have you have CZLB enabled then the traffic is distributed evenly across all instances regardless of AZ. If it is disabled traffic is distributed evenly among AZs, if you have two AZ it’s 50/50, 3 it’s 33/33/33, etc. Then those AZ split the traffic amongst the instances, so if you have an AZ with 10 instances and another with 2 both AZ will receive 50% of the traffic despite the number of instances meaning that the AZ with less instances is under a heavier load. With an ALB if you have CZLB enabled cross AZ traffic is not charged, however it is charged on NLB and GLB which is partly why it’s disabled by default.