Wifi extenders don't correspond unique signals. Instead, they work with the existing signals in the network. They need to be placed where the network is strong since they are intended to extend it to a specific location. Unlike wireless access points, they are not used in dead spots or add strength to a weak Wifi.
However, access points are networking devices used in large buildings or business organizations. They're usually connected to a router or switch through a wired connection. They extend Wifi signals to designate areas or to reinforce network signals. For example, if you want Wifi access in your living room but don't have a router there, you could set up an AP and run the Ethernet cable through the ceiling back to your central server/hub.