Intersystem Communication (ISC) provides communications between two MSS systems, or between a CICS systemand MSS, or between a CICS system or MSS and an IBM-compatible CICS system using Mainframe Access. It enables distributed processing, where the processing of CICS transactions is split between various systems.
You need to do some configuration to get these facilities working. Some of this is common to all distributed processing facilities, while some is specific to one facility.