You would use monX for monitoring alias's.
monX as apposed to wlanX. wlan0 should only be used to identify a specific wireless ethernet interface where as mon0 should be used when a interface is mapped to a program function. Alias's can be bound to two or more different devices for example when using host mode. Or when using two or more different configurations of drivers/settings or both/more of the earlier.

ps: distros are also moving away from using device specific interface alias's and only using them for debugging as well.