I think it means you need at least 8GB of space, think the extra space is for patches and updates.....maybe wrong but thats what i think
But most of the time this happens because it needs the remaining memory for Windows Swap File. So it needs 8GB free space, it need not necessary occupy all the 8GB after the game is installed.
* Updated game trainers and cheats daily
* Get notified when new cheats are added
* Request which games get new trainers
* Priority support with any problem