As with all Fair Usage Policies, if you use more than a pre-determined ‘fair’ amount of data in a given timeframe you may stand to be restricted to slower line speeds during peak times:

Service Plan Name First Throttle L1 Second Throttle L2 Third Throttle L3
Unlimited 20 Mbps 200GB 7Mbps 350GB 3Mbps 400GB 512kbps
Unlimited 10 Mbps 150GB 3Mbps 250GB 2Mbps 300GB 512kbps