You will notice the "Sort by" option on the top left (above the results but below the search bar) on the 'Review' page.
When sorting by Family (as displayed above), your documents will list a "Parent" document first immediately followed by the "Child" (or children) document(s). Next, children of the children are displayed. A "Parent" is any document that has an attachment, like an Email for example. A "Child" is the attachment, such as a Word File. Also, every document when sorting by Family is sorted by "Last Modified Date".
In the image shown above, you see an Email (the "Parent") followed by two "Children" documents (attachments). If you open to review the "Parent" you will see the two "Children" listed under the "Attachments" tab to the left of the document and above the Metadata.
The "Children" documents are sorted by Sent Date from earliest to most recent, top to bottom.
How Sorting By Family Affect Documents Without A Child Or Parent
Independent documents are also sorted by "Last Modified Date". These independent documents are treated as a "family of one".
Sorting by Family For a Production
To learn about the advanced options for sorting by family during a production read this article.