How do instructors respond to a student's private question in Piazza?
If you allow the default permission for students to post private questions to an instructor (See: How do I edit the student anonymity settings for my Piazza site?), students can sent the instructor private questions or notes. The only users that can see a private question or note are the instructor and the student that posted the private question.
Note: Students can direct the private question or note to an individual instructor.