You can specify that unused sectors of the source drive be overwritten with zeros under Clone drive/Cloning options/Advanced/Clear unused sectors from old data. By enabling this function, you will not be able to restore data of the empty areas from the drive clone.
This function will be run during the next clone activity of the selected drive.
If you delete data normally under Windows, it still remains intact on the hard disk. The data will no longer be accessed by the operating system and remains invisible for the user. The space is declared as free area by Windows. This data can still be restored using special software. To make sure that nobody can restore data from the drive clone, enable the function Clear unused sectors from old data. After overwriting, not even specialized data recovery software will be able to restore this deleted data.