One of the less talked about new features introduced in iOS 9 last month is called "Wi-Fi Assist," which automatically switches a user's iPhone to a cellular data connection when the local Wi-Fi isn't reliable.
Many iPhone users have taken to Reddit and the MacRumors Forums to warn others about the possibility of an increased monthly data plan due to the new feature, but now Apple is getting out in front of the issue and attempting to ease the qualms of its customers with a new support document centering on the issue (via Six Colors).
According to Apple, when a user sees the switch from Wi-Fi to cellular data while browsing an internet page in Safari, for example, they should only see a "small percentage" hike in data usage. The company also detailed which apps support the feature -- Apple Music, Mail, Maps, Safari, etc -- and still gave those concerned a step-by-step process to turn off Wi-Fi Assist. It ended the new support doc with a bullet list of interesting tidbits about the feature.
-Wi-Fi Assist will not automatically switch to cellular if you're data roaming.
-Wi-Fi Assist only works when you have apps running in the foreground and doesn't activate with background downloading of content.
-Wi-Fi Assist doesn’t activate with some third-party apps that stream audio or video, or download attachments, like an email app, as they might use large amounts of data.
Wi-Fi Assist is supported on any device that runs iOS 9 or later, excluding iPhone 4s, iPad 2, iPad (3rd generation), and iPad mini (1st generation). If the support document doesn't entirely ease your concerns, go to Settings > Cellular, and scroll down to underneath the full list of apps that use cellular data to find Wi-Fi Assist's on/off toggle.