There are components that run on the USS side of things and mainframe side of things.
The USS file system doesnt have to be shared.
The GUI component is the only portion of the product that runs on the USS side of things. The rest runs on the mainframe.
Usually the GUI is run on one instance of the multiple LPARs/CPUs and communicates to multiple LPARS/CPUs via CA LDAP.
You could run multiple GUIs if you wish, if you need a redundant GUI with the additional cost of directory space and CPU usage.