When following the Package Principles or traditional modular architecture there exists no layers. Actually modular architecture is often seen as an approach to going away from the traditional 3 tiered architecture (UI, business logic, data). Layers in the Habitat methodology is not this kind of layers.
The layered approach in Sitecore Habitat is aimed at tightly controlling dependencies between modules and thereby is a vital part of the architectural approach.
The structure is split into three layers: Project, Features and Foundation, which all consists of modules.
- The Foundation layer consists of services which are common across the whole or parts of the solution.
- Features - or the functionality of the solution - is built as modules in the Feature layer.
- The unified solution, websites or channels, are brought together in one or more Project layer modules.