When a request first comes in from an HTTP client to the load balancer, it is a request for a new session. A request for a new session is called an unassigned request. The load balancer routes this request to an application server instance in the cluster according to a round-robin algorithm.
Once a session is created on an application server instance, the load balancer routes all subsequent requests for this session only to that particular instance. A request for an existing session is called an assigned or a sticky request.