A normal question: "What happens if I exceed the amount of bandwidth I have agreed to pay in my unmetered services contract"? Though it should not happen, for your consumption if being monitored, if for some reason you exceed the amount of bandwidth you have agreed to pay for, "your traffic will be shaped until it falls back into your allotted bandwidth", (webhostingtalk.com).
Traffic is categorized in three levels - 1) sensitive, 2) best effort and, 3) undesired. The "shaping of traffic", also known as "packet shaping" or, "ITMP's" (Internet Traffic Management Practices) is "the control of computer network traffic in order to optimize or guarantee performance, lower latency, and/or increase usable bandwidth by delaying packets that meet certain criteria." (IETF RFC 2475 "An Architecture for Differentiated Services" section 126.96.36.199 - definition of "Shaper"). Unmetered services, therefore, takes your undesired traffic and, in layman's terms, puts it at the back of the line to wait until the bandwidth returns to its normal range. Companies which have offices in separate geographical locations are preferring to shape their traffic instead of increasing their wide area network, thus saving money.
Due to the fact that this is a relatively new industry there is not much regulation or industry standards to use as guidelines. Therefore, different companies will employ their own interpretation. Make sure that when signing any contract, you read such carefully - especially, the fine print.