The domain model, services and repository interfaces. This is the central part of the application. The ubiquitous language is used in classes, interfaces and method signatures, and every concept in here is familiar to a expert in the cargo shiping domain.
There is no infrastructure or user interface related code here, except for things like transactional and security metadata which is likely to be relevant to a domain expert ("Either all of foo succeeds or none of it does", "In order to do bar you need to be a Supervisor", and so on).