Traffic has to be booked in advance, i.e. provided your server shall not be throttled when reaching the exemption limit you have to book extra traffic before reaching the traffic limit stated in the tariff. 

An example:
You are booking an extra traffic of 5TB for your contract (which already includes 20TB) at the beginning of the month.

Result:
You can use the cumulated sum of 25TB within the server contract in the current month without being throttled in bandwidth. Without booking the extra 5TB you would already be throttled at 20TB. If the 25TB have been reached on the 20. day of the current month for instance you aren't charged for the extra traffic but throttled down to 10Mbps. If you're booking another 3TB to "make do" on the end of the month the bandwidth restriction is suspended until you're reaching the end of the booked volume again. 

Please keep in mind:

- Booked extra traffic will be carried over to the next month.
- Starting with higher traffic volumes a traffic flatrate could be more favourable than booking the traffic TB for TB. Contact our sales departement in this case!