Final answer:
Configuring a value from the ID drop-down list while setting up a WLAN in the WLC GUI determines the unique ID on which the WLAN will operate. This ID is used by the wireless controller to uniquely identify the WLAN.
Step-by-step explanation:
When selecting a value from the ID drop-down list box in the WLC GUI when configuring a normal WLAN, you are configuring the unique ID on which the WLAN will operate. This ID is critical, as it serves as a unique identifier for the WLAN within the wireless controller to distinguish it from other WLANs that may be configured on the same controller. The ID does not represent the type of WLAN, the network name clients will use (SSID), or the WLAN's configuration name within the WLC GUI, although all these aspects are essential parts of the overall WLAN configuration process.