Added in iOS 9, Wi-Fi Assist recognizes when you’re connected to a Wi-Fi network, but have a poor or erratic signal. When this happens, Wi-Fi Assist kicks over automatically to cellular for foreground apps to keep data flowing. This feature is enabled by default.
Wi-Fi Assist automatically excludes third-party audio and video streaming, and doesn’t relay data from apps working in the background. Nonetheless, some users wound up with surprising cellular bills when they thought they were on Wi-Fi networks—it turned out Wi-Fi Assist used cellular data because the Wi-Fi connections were marginal. (U.S. carriers have largely switched to a fixed or flexible monthly maximum after which your cellular data rate is throttled to a very slow speed, so surprise bills are less likely.)
