Nostromos business logic is based on modules. A module consists of its own data model, a controller class and several views. Slicing of modules is based on the domain. A module is comparable to how one would slice a microservice.
Some of the modules are required by the framework for basic operations as the user module or the CMS module. Others are truly optional. Modules that can be shared with multiple other projects reside in the Nostromo or NewFrontiers namespace while project releated modules are in their own namespace. For older projects this is mostly the project name (like Osiris for the API Gateway Manager), newer projects mostly use the standardized App namespace in line with other PHP-based frameworks.