No data plan is really unlimited.
If you start to go over a certain amount of data 10 GB like I did with Sprint unlimited data then they throttle you back to a slow speed and if you continue to go over the 10 GB they will terminate your service.
At least that's what happened to me a year ago. Not sure if things have changed or not but I doubt it.
If you read your Terms of service agreement they will list an amount of data you're actually allowed to use.