Bandwidth consumption is more complicated than what can be described with just a single number. However the exact scenario you are asking about can mostly be explained by looking at two measures.
What is the actual link speed and what is the monthly quota?
Link speeds are usually measured in megabits per second (Mb/s for short). Typically those ranges from 10Mb/s to 1000Mb/s for a VPS. The link speed is never unlimited since there are physical limits to what the hardware can do.
If the link speed is fully utilized all the time, we can compute the monthly consumption by a simple multiplication. 10Mb/s will come out as roughly 3TB/month if fully utilized. But almost everybody have needs that wary over time, and tend not to use nearly the total amount of capacity permitted at the link speed.
This is why some provides will advertise 100Mb/s or even 1000Mb/s with a monthly quota, and they may enforce the monthly quota by reducing the link speed to 10Mb/s in case the quota is exceeded.
Operating with no monthly quota is certainly possible, especially if each customer is given only 10Mb/s link capacity or if the provider can reduce the link capacity with no notice.
Any offer claiming unlimited data transfer will at best mean there is no monthly quota. When evaluating such an offer you have to look carefully at the link speed to decide whether it is sufficient for you. There will be a limit, and if the provider won't tell you what the limit is, you should be worried.
There is also the possibility that the provider says one thing but in reality operate with some unwritten rules. The conditions likely have very broad definition of what is unacceptable usage such that the provider can always get out of the contract should they find usage to be too high.
