Wipe available diskspace

[Narcis Garcia <informatica actiu net>]

I'm using nautilus-wipe extension, and today I wanted to look the "Wipe
available diskspace" function.
In all phrases and in the manual only use the term "available" about
space, and I need to be very sure this is referring exclusively to
unallocated blocks, it is, space not assigned to any inode nor
filesystem working structure.

Help should tell more about what means wiping available diskspace.