Organizations and groups are ways of categorizing users. Users belong to groups and groups belong to organizations. An organization and group coupled together is frequently referred to as a domain and is considered unique.  That is to say, a group can belong to more than one organization but an organization and group together (or domain) is unique.  For example, you've created the following groups

• Engineering
• Sales
• Marketing
• Customer Service

These are generic groups that can apply to any number of organizations.  Rather than having to create a new group of the same name for every organization, you can share these groups' attributes.  Now let's say you have two organizations:

• Johnson Supply Inc.
• InFocus Group, LLC.

Each has the following groups:

• Johnson Supply Inc.
• • Sales
  • Marketing
  • Customer Service
• InFocus Group, LLC.
• • Engineering
  • Sales
  • Marketing
  • Customer Service

With that hierarchy in mind we can create the following domains:

• Johnson Supply Inc. / Sales
• Johnson Supply Inc. / Marketing
• Johnson Supply Inc. / Customer Service
• InFocus Group, LLC. / Engineering
• InFocus Group, LLC. / Sales
• InFocus Group, LLC. / Marketing
• InFocus Group, LLC. / Customer Service

In Nexus, you have created a total of four groups only and two organizations. 

Why Do Domains Matter?

Domains are an important part of Nexus because it is through domains that permissions to individual files and categories are assigned.  Rather than allowing user A or user B to see a particular file, you allow an entire domain to view that file.