Domain layer in Extbase 

The domain layer is where your extension's data lives. It consists of two things: models, which are PHP classes that represent your data, and repositories, which are the only entry point for reading and writing that data to the database.

A model describes what an object looks like — its properties, their types, and how they relate to other objects. A repository knows how to find and persist those objects. Controllers and views should never touch the database directly, but refer those tasks to a repository.

Value objects — objects defined by their value rather than their identity — are also part of the domain layer. In TYPO3 v14, value objects are implemented as plain PHP classes rather than by extending an Extbase base class. Their usage is covered in the model page.