This has been the de facto solution for load balancing and is backward compatible with the EC2—classic networking mode. This is an Open Systems Interconnection model (OSI model) level 7 load balancer capable to handle communications with HTTP/HTTPS, TCP, and SSL. The CLB can map different ports for the listener and the destination target. For example an HTTPS listener on port 443 and a target with HTTP on port 80.
The following flow diagram will help you to decide when to use Classic Load Balancer (CLB):
This chart is not comprehensive and there are characteristics that are shared between load balancers.