Sub Apps allow developers to create multiple apps each with distinct names, icons and OS Integrations under a single Isolated Web App (IWA) installation. Windows of sub apps appear on the shelf with their specific identity, distinct from the parent app. Sub Apps are deeply integrated with the OS and offer capabilities such as file type association. This enables a single software package to represent different, isolated functionalities to the user while maintaining a single, unified IWA install and update process.
Without this API developers are forced into splitting the application into separate ones which complicates distribution or bundle everything into a single huge application that can potentially confuse users.
Docs: https://github.com/explainers-by-googlers/sub-apps
Samples: https://github.com/vkrot-cell/subapps-demo
Explainers: https://github.com/explainers-by-googlers/sub-apps