An application server is a multithreaded CICS® process that provides a complete environment for running a CICS transaction. If your transaction is conversational, it will be processed by exactly one application server. If your transaction is pseudoconversational, each transaction in the sequence might in turn be executed by a different server.
When you configure CICS, you specify the minimum and maximum number of servers for your region. The transaction scheduler ensures that this minimum number is always present, and creates and destroys servers up to the defined maximum number, depending on the overall workload.
All the application servers in the pool have to be located on the same node, but do not need to be on the same node as the CICS on Open Systems client is. They communicate with each other by using signals and shared memory, and use RPCs to communicate with the other components of the region.