| This is a priority mechanism described in the man page apt_preferences and the apt howto.
This says that testing will be used by default, and you will be able to install specific packages from unstable by using “apt-get -t unstable install package”. And, if you set the pin priority to something bigger than 1000, when you will do apt-get upgrade, it will actually downgrade your system to Woody if you had any packages from unstable. This can be handy to go back to a clean Woody. From that point, you can selectively install packages you want from unstable.