The client is the part of the client-server architecture through which you launch a simulation and work with it, typically through the STAR-CCM+ workspace.
You can see client processes running on your computer (using ps in Linux or the Task Manager in Windows) when a simulation is open and running locally in serial mode:
java in their listings.
starccm+.exe, while the GUI-based process has the name starccmw.exe.
Tasks performed by the client include starting the simulation, inputting commands and manipulating views for analysis. However, it is possible to load a simulation without starting the client.