In master controller mode, the Wireless LAN Controller (WLC) that is designated as the master will be the one that all new Access Points (APs) that join the WLAN will associate with, provided they have not been pre-configured with primary, secondary, or tertiary WLCs. This mode is particularly useful in scenarios where there are multiple controllers within the same network and a new AP has to select a controller to associate with. The master controller mode simplifies the process by ensuring that all new APs without specific controller assignments will associate with the master controller.Reference: https://www.cisco.com/c/en/us/support/docs/wireless/4400-series-wireless-lan-controllers/6