Some apps depend upon background data (transferring data when not directly in use). Restricting background data may cause those apps to stop functioning unless a Wi-Fi connection is available.
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From a Home screen, tap Apps (located in the lower-right).
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Tap Settings.
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From the Wireless and network section, tap Data usage
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Tap an app (located below the usage graph).
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Tap Restrict background data.
Enabled when a check mark is present.
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If presented, review the message then tap OK.
Click here for info on setting data limits and warnings, changing the usage cycle on your device and viewing data usage by app.