An NIS+ domain consists of a directory object, its org_dir directory, its groups_dir directory, and a set of NIS+ tables.
NIS+ domains are not tangible components of the namespace. They are simply a convenient way to refer to sections of the namespace that are used to support real-world organizations.
For example, suppose the DOC company has Sales and Manufacturing divisions. To support those divisions, its NIS+ namespace would most likely be arranged into three major directory groups, with a structure that looked like the following diagram.
Instead of referring to such a structure as three directories, six subdirectories, and several additional objects, referring to it as three NIS+ domains is more convenient.