p. 50 said:
Setting Up a Wireless Distribution System (WDS)
When you connect base stations wirelessly in a WDS, you set up each base station as either a main base station, a remote base station, or, if you are using AirPort 3.1 or later, a relay base station. Note: If you are setting up AirPort Express to extend the range of your network using WDS, use the AirPort Setup Assistant. See Extending the Range of an Existing AirPort Extreme or AirPort Express Network on page 70.
A main base station is connected to the Internet and shares its connection with remote and relay base stations. A remote base station shares the main base stations Internet connection. A relay base station shares the main base stations Internet connection and transfers the connection to other remote or relay base stations. All three base station configurations (main, remote, and relay) can also share the main base stations Internet connection with client computers wirelessly, or with Ethernet if the client computers are connected to the base station or Ethernet. When you set up base stations in a WDS, you need to know the AirPort ID of each base station. The AirPort ID is also known as the MAC address and is printed on the label on the bottom of the base station next to the AirPort ( ) symbol. To make it easier to set up a WDS, place all of the base stations on a table and plug them into a power supply. As part of the WDS setup process, you might consider giving all the base stations unique names, to make them easier to identify in the future.