What is a Layer?

Layers build the framework of a Roadmap. They are used to separate sets of Activities thematically.

Layers build the framework of a Roadmap. They are used to separate sets of Activities thematically. Layers that are dependent on each other can be split into Sublayers.
A basis to start from can be the Why-What-How principle: main layers and their Sublayers that fit into each category should describe the market pull (trends, drivers, barriers, etc.), product/service range (offering) and technology push or resources.