Recently quite a few carriers have started putting caps on their data usage, most of which are set to 5GB. When you go over this cap, you’re usually charged overage fees, which can add up very quickly, especially for multimedia phones and smartphones. Now, instead of charging overage fees, T-Mobile will be throttling your connection speed once you hit the cap.
“Subscribers with a 5 GB per month webConnect plan will no longer be charged per megabyte overage fees, but will simply see their connection throttled. This should mean while entertainment use is limited, IM and email should work perfectly fine.
Subscribers on the 200 MB limit, who arguably need this much more, are unfortunately not as lucky, and will still have to pay overage, but at a lower rate of $0.10 per MB instead of $0.20, which can still add up pretty quickly.”
Source: WMPowerUser