This should be pretty quick to answer: is the optional essay the same thing as a diversity essay? I realize there's probably not a blanket answer for all schools, but both Boston College and Boston University have optional statements that they describe as:
"You may upload an optional statement, regarding any economic, cultural, social, or other factors that you wish for us to consider when reviewing your application." (BC)
"In addition to your personal statement, you may wish to provide another essay. This essay is your opportunity to discuss any aspect of your background or life experience that you believe will enhance your ability to contribute to the diverse BU classroom experience and community. BU Law values and recognizes the importance of diversity. BU Law continues its long-standing tradition of providing opportunities for persons of all backgrounds and providing the excellent training to which a diverse classroom is indispensable." (BU)