Overselling enabled is where we take the amount of disk space and bandwidths ACTUAL USE between all your accounts.

For example, if you had the 5 GB space and 50 GB transfer plan and you decided to offer plans such as 1 GB space and 10 GB transfer on your site, you would only be able to offer it to 5 clients.

With overselling enabled, you would be able to offer as many as you want as long as the aggregated amount USED does not go over the 5 GB space and 50 GB transfer.