On an IBM mainframe, when a CICS application accesses a VSAM dataset, the access is transactional. This means that CICS, acting as a Transaction Manager (TM), ensures that the updates to the VSAM dataset are committed or rolled back as appropriate. The application can control the transaction boundaries with explicit EXEC CICS SYNCPOINT commands, and CICS implicitly controls the transaction at end of task.
In Mainframe Subsystem Support, CICS provides VSAM emulation but does not provide transactional access to these resources. Fileshare does provide such access. Mainframe Subsystem Support acts as the TM and uses Fileshare's Resource Manager (RM) to provide behavior compatible with mainframe CICS.
No changes to your program source are needed to take advantage of Fileshare. If a resource definition indicates that a resource uses Fileshare, the default configuration provides backward recovery for that resource. This means that if an application explicitly backs out updates to a VSAM dataset, or CICS rolls back an update because of an application failure, Fileshare ensures that the dataset is returned to a consistent state.
If a file is accessed only from JCL, not CICS, then there is no advantage in using Fileshare for it. However, if it is accessed by both, and you configure it to use Fileshare for the sake of transactional VSAM access, then Fileshare is used for all accesses to it, whether from CICS or JCL. The later chapter Using Fileshare - More Features describes how to handle potential conflicts between accesses from CICS and JCL.