Product Mapping and Discovery Trees were tools that were discovered when FAST emerged. They helped visualize work, track it, and create a shared context and understanding. The discovery trees really helped with Fluid Teaming as an incoming team member could very quickly get a picture of the work and the context it sat in.

While these tools are described as being part of FAST in the 2.12 version of the FAST Guide and earlier, from version 3, they will be useful additions or framework aspects of FaST and not a part of the core.

Some Blogs on Discovery Trees

Discovery Trees - A Simple and Natural Way of Story Creation, Project Tracking, and Shared Ownership by Quin Bill

Working With Discovery Trees by Steve Kuo

Why Discovery Trees Outshine Jira: Simplifying Visualization and Collaboration
by Erik A. Ekberg

And a couple of videos...