I can't seem to find documentation on how wifi is used vs cellular data. It would seem logical that wifi will be used whenever it is available and the application, or network stack, will switch to cellular data if wifi fails. However, where can we get an authoritative answer on this? When will cellular data be used? Only if there is no wifi connection? What if there is a wifi connection, but there is no general Internet access, for example when you are connected to one of those services that require a manual login first? After how many times when wifi fails or for how long after wifi fails before cellular data will be used? Thanks.