For convenient ref on how Data Sense works...
Use Data Sense to track data usage | Windows Phone How-to (United States)
There were some changes/additions that came with the GDR3 update (Windows Phone software version 8.0.10501.127 or later)...
Data Sense can reduce the amount of background data you use. You can choose when to restrict background data:
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'When I'm near my limit', you can choose to restrict background data [GDR2}
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'Always restrict background data' [GDR3]
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'Restrict background data when roaming', [GDR3]
Additionally, the 'Reduce data use browing...' switch, which in GDR2 was tied/accessed via IE app, is now available in the Data Sense app settings..which as previous, sends URLs to Data Sense to reduce data usage. I'm not sure if not being tied to IE allows this feature to work with other wp8 browsers or not eg UC Browser, Touch Browser etc. If so, that would be a nice improvement if not already enabled in GDR2..
Important caveats to not create other issues...
- Restricting background data
may prevent some things from happening automatically, including voice over Internet protocol (VoIP) incoming call notifications, email syncing, background uploads and downloads, and Live Tile updates. All depending on your choices of restriction of data useage.
- Selecting
Always means that your Live Tiles won't update, and you'll need to manually sync your email to get new messages. So in most cases, you'll want to use 'When I'm near my limit'.