Parent documents are either the parent email to an email attachment, or the parent document of an embedded document (e.g. an Excel spreadsheet embedded in a PowerPoint presentation).
Click the “Parents” button to list all parent documents of the current document. In this example, the current document has three parent emails - likely because the attachment was duplicated in three different emails. The following information is displayed for each: DISCO ID, filename, and an excerpt of first few words of the parent document.
By default, when you tag a parent document, all of the parent's children (e.g. attachments) are tagged.