Answer: By default,when you tag a parent document in Disco, any child of that document will also get tagged. For example, if I am viewing a parent email and tag the email "hot" within the document viewer, the email's attachments (children) will also get tagged "hot". To propagate a tag UP from a child to it's parent, you must mass tag (see screenshot below). Read more about how to mass tag HERE.
If you would not like Disco tags to automatically propagate down from the parent to it's children, contact Disco Support to discuss the benefits and drawbacks of turning off this matter-specific feature.
*Please note, this information holds true for all matters created before 12/06/2016. For matters created on or after 12/06/16, please read Tag Propagation.