A client-Server System consists of one or more client computers and one or more server computers. In case of having a patent of a "system", if a 3rd party only uses the same client computer as the one in the system, the 3rd party may not infringe the patent.
An invention with a client-server model should be broken down to a client machine and a server machine, and a patent claim should be drafted for each of inventions of the client machine and the server machine.