understanding dmsetup

[Anders Ingemann <anders@ingemann.de>]

So right now I am trying to install grub the proper way on a loop device.
To that end I am following this guide: http://ebroder.net/2009/08/04/installing-grub-onto-a-disk-image/
I understand pretty much everything with the exception of the last '0' in the table info that is passed to `dmsetup create hda'.
As far as I can see the table consists of the following:
echo '0 20971520 linear 7:0 0' | sudo dmsetup create hda
      |    |       |    | | |
      |    |       |    | | +--- ???
      |    |       |    | +--- minor device number
      |    |       |    +--- major device number
      |    |       +--- target type
      |    +--- num sectors
      +---start sector

What does the last '0' do?!

Anders