In client mode, the AirPort Express is used exclusively by the device to which it is physically connected. Additionally, client mode can be used for AirPlay music streaming from a computer running iTunes to a sound system. To use any of these features, you will also need the appropriate Ethernet, audio, or USB cable to connect the device to the AirPort Express. Then configure your extended Wi-Fi base stations, ensuring that each is within direct range of the primary Wi-Fi base station. Select "Add to an existing network". Before you begin, make sure that your AirPort software is up-to-date.
Additionally, client mode can be used for AirPlay music streaming from a computer running iTunes to a sound system.
AirPort base stations: Setting up and configuring an extended wireless network (802.11n)
Only compatible USB printers or audio devices are supported. From the Radio Channel Selection menu, select Automatic. Choose the network you want to extend from the Network Name menu. The physical location of extended Wi-Fi base stations will vary according to the building environment and may require some experimentation. Setting up and configuring an extended wireless network