Domains are the components of the Analysis of Application realm. The top level domain is called the application domain of which there can be more than one.
Bridges are the interfaces between domains.
Domain and Bridge Example
In a project the application domain is shown with all the other domains in the application. Bridges to other domains are effectively "channels" that a domain sends events to and receives event from.
Domains with Central Entities Displayed Example
It is often useful to name the major entities comprising a domain and to have them visible on the DBD. The domain entities shown are usally the central and active entities.