The shared library region is a z/OS feature that enables address spaces to share dynamic link library (DLL) files. This feature enables your CICS regions to share the DLLs that are needed for JVMs, rather than each region having to load them individually. This can greatly reduce the amount of real storage used by MVS, and the time it takes for the regions to load the files.
The storage that is reserved for the shared library region is allocated in each CICS region when the first JVM is started in the region. (This might be the master JVM that initializes the shared class cache.) The amount of storage that is allocated is controlled by the SHRLIBRGNSIZE parameter in z/OS. “Tuning the z/OS shared library region” in the CICS® Performance Guide tells you how to tune the amount of storage that is allocated for the shared library region.