The DNSDomain property indicates an organization name followed by a period and an extension that indicates the type of organization, ...

The DNSDomain property indicates an organization name followed by a period and an extension that indicates the type of organization, such as microsoft.com. The name can be any combination of the letters A through Z, the numerals 0 through 9, and the hyphen (-), plus the period (.) character used as a separator.
Example: microsoft.com