WHAT IS A FIELD, A SUBJECT, AND A CONCENTRATION AND HOW DOES THIS RELATE TO SPECIAL COMMITTEES
In the Cornell Grad School, a major Field is a group of faculty that administer a graduate degree program (see this page). Fields can be partitioned into Subjects. Most fields have only one subject and the distinction between Field and Subject is uncommon for PhD students, so most people have never heard or thought about Subjects (see the complete list here–one example of a Field with multiple subjects is Evolutionary Biology and Ecology, which has an Ecology Subject and an Evolutionary Biology Subject).
The Mechanical Engineering Field’s one subject is called Mechanical Engineering (creative!!). Each Subject is partitioned into concentrations. Mech Eng has 7 concentrations: biomedical mechanics, dynamics and control, energy and sustainability, fluid dynamics, micro- and nanoscale engineering, solid mechanics and materials, and thermal science.
The graduate school specifies the constituents of a committee in terms of major and minor Subjects. Because most Fields have only one subject, most people describe committee requirements in terms of Fields as shorthand.
A minor Field is a Field that offers no admission and no degrees, it serves only to provide minor Subjects. Faculty can represent those field as minor subject members, but not as Special Committee Chairs. The only minor Field I can find that was likely relevant to Mech Eng PhD students is Computational Science and Engineering.