As mentioned in the other answer, constructed lists allow you to provide specific instructions on how a list of answers should show up in a question, i.e. only show items that were checked in a previous questions.
You can read more about list building at www.sawtoothsoftware.com/support/issues/ssiweb/online_help/hid_web_listbuilding.htm.
Additionally, you can use custom Perl code if you wish to do something that our predefined functions (thinks like add if checked, add if rated greater/less than N, etc.) don't quite meet your needs.