The pietree provides an overview of Treejack results and lets you see, at a glance, where an IA works and where it doesn’t. It shows which way your participants went at each junction in your content hierarchy and what they all selected as their final answers.
You can access the pietree from the Task Results tab. One pietree represents your participants' responses to one task, so you'll be able to see one pietree per task.
This key shows you what each color represents. You can see this key by hovering your mouse over different parts of the pietree.
Benchmarking a pietree from your existing structure against a proposed new design is a great way to validate that your information architecture. This will help you to ensure your IA and content labels are ideal for your users before you move on to wireframing and visual design.
Check out this poster that we made for a conference. It'll give you some more insights into how pietrees work.