Whenever you purchase a reseller account, you are not allowed to oversell your traffic; this means that once you have divided your available traffic amongst your customers, you will no longer be able to create new accounts.
If you purchase the overselling upgrade, you will be allowed to sell as many accounts as you want, with no limits to the traffic you give to your clients; we will only check that you do not effectively go over your limit at the end of the month.
Example:

Your reseller account comes with 60 GB of monthly traffic; you can sell 100 GB or even more, but as long as you do not go over the 60 GB of dataconsumption, we will not charge you for it.