Originally posted by MrAdmin that's mean i have to unpick this option
Ticking it would be preferred.
For example you give 5GB of space to a reseller and 35GB of BW.
The reseller can create accounts which have 10GB of space and 100GB of BW. But when the account goes over the reseller limit (5GB and 35GB BW) then they can't access the site anymore (if they used 35GB of BW) or they can't store anymore files (if 5GB is used).
I would personally ALLOW overselling as if a scenario like above happens then your client is most likely to buy a lot more BW and space from you after they run out.