As illustrated in the image below, bandwidth (yellow) is ‘pooled’ on the server between all accounts. Users 2 and 3 went over their allowance but the pool is still below the server allowance, so nobody pays any overage charges for the month.
Example 1
In this example, the server pool exceeded the maximum allowance, so users 2 and 3 may potentially have overage charges due for the month. However, users 1 and 4 are under their allowances so they will not be billed for overages.
Example 2