In the Cooperation Tool, Domains are specific sections of the repository where Objects can be created and retrieved. In other words, Domains are for Objects what folders are for files in your PC.
The structure of the Domains can be defined so as to perfectly match the structure of the Project.
This facilitates to know where an Object is located within the domain structure.

Choosing a Domain, your view on the Project Objects is limited to those Objects, which have been created in such Domain and, optionally, its sub-domains.