I am not a trained singer so the concept of resonance is relatively new to me. The following statement provided a lot of clarity for me: "...a singer who has good resonance is more efficient in converting breath energy to sound...". This book does a good job of putting difficult-to-explain vocal phenomenon into words.

I am unclear on how to define "upper register" and "lower register". Are these terms relatively ambiguous, or are there certain pitches that fall into the upper and lower categories for each choral section?

Davids, J., & LaTour, S. A. (2012). Vocal technique: A guide for conductors, teachers, and singers.