jCore client is the stripped version of jCore server. There are no libraries nor icons separately installed for the jCore client however the lib/ directory is allready created if you have different (customized) libraries for different clients. Just copy over the library file from the jcore server and start editing the code, because that code will only be used by that client and none of the other clients nor the server's functionality will be affected.
The same applies to the modules, there is a lib/modules directory already created, if you want to install a module only for a client just copy the module into that directory and you are ready. The same goes for the libraries, if you like to customize let's say the photo gallery module, just copy the module over and start modifing it's code, this will only affect that client.
To learn more about howto install modules please see the Help -> Modules section.
jCore client is usually in every client's home directory, for e.g. /home/client1/public_html/ so no code is shared between the clients, nor symlinks are made to the core allowing you to freely give ftp/sftp access to your clients if asked, not having to think about that they may see other clients files/codes. They wont!
Te learn more about how to install jCore client on your server please see the Help -> Installer section.