You use the vol offline command to take a volume offline to perform maintenance on the volume, move it, or destroy it. When a volume is offline, it is unavailable for read or write access by clients.
When you take a FlexVol volume offline, it relinquishes any unused space that has been allocated for it in its containing aggregate. If this space is allocated for another volume and then you bring the volume back online, this can result in an overcommitted aggregate.