The assembly that implements the domain of your application, i.e. contains your subclasses of DomainObject (or, BindableDomainObject). Such subclasses are called domain object classes or domain classes.
The domain assembly can be inspected by dbschema.exe to derive a database schema for persisting instances of your domain classes.
The domain assembly can be inspected by uigen.exe to discover your domain model (http://en.wikipedia.org/wiki/Domain_model) and derive a functional web application from it.