The Release Map or Product Wall is based on the idea of story mapping. It is a way to visualize the entirety of the backlog quickly.
It is initially populated with features that are not broken down. When and how they are broken down is up to the tribe and is work and done by a swarm inside an iteration. The broken-down work is represented in their own feature maps.
Breaking work down into consumable chunks
Like everything else in FAST, work break down happens in a just enough, just in time fashion. Features are broken into large stories, and then again into smaller and smaller work items until they are consumable by a team. An example is:- a swarm steward decides to work on a new feature. The freshly congregated team need to determine what is a sensible thin slice that they can deliver in the iteration with the number of members and skill set in their team room. To this end, they will begin the breaking down of stories and stop when they have found the most sensible items that meet these requirements. This would be the first activity for a team to do before any coding in the case of a new feature or a large item that hasn't been broken down yet. These work items are posted onto a feature map which becomes more and more fragmented and fine-grained as the project proceeds.
The features/stories/items may or may not be sized. That is up to your tribe if you choose. FAST can work in a sized/estimated or no-estimate environment.