Product Mapping and Discovery Trees
Product Mapping and Discovery Trees were tools that were discovered when FAST emerged. They helped with visualizing work, tracking work, and creating 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 it.
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 best practices or useful additions and not a part of the core.
Infographic of FAST Core and Supporting Components
Some resources on the topic of Discovery Trees and Product Mapping:
Quin Bill Blog - Discovery Trees - A Simple and Natural Way of Story Creation, Project Tracking and Shared Ownership
Steve Kuo Blog - Working With Discovery Trees
And a couple of videos below: