I believe that ONLINE refers to that action when Viber is opened and the user is using it to chat or call. I've looked on various websites and found the same information regarding the online availability. Some users installed 3rd party apps that faked their status, therefore, you can't always know what is the exact status.
It's also impossible to contact Viber to ask them about this because in the luckiest case possible, they will reply and forward you to one of their pre-defined FAQ pages.
