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.