Just to add to the 'Dawgs white paper on the subject
1) For purposes of LOR/LOE, what is science?
Many applicants equate "science" with AMCAS "BCPM". This is not accurate. Science for purposes LOR/LOE is what "normal" science would be at a college or university. This would include Biology, Chemistry, and Physics, certainty. But it would also include Oceanography, Geology, some Psychology or even Computer Science and Materials Sciences. Under this general theme most health and medical science fields could be considered science. What about engineering and math generally? They can actually be framed either way, which usually depends more on the background of the candidate than anything else (ie what professors has the applicant worked with.).
2)How can I judge if a letter writer fits a science or non-science criteria?
Applicants who need to look at this issue must do something that few seem to do: use their judgement. You can look at the degree the letter writer holds, the department to which they are faculty, and the title/position they hold within that department. Perhaps even more telling is the course that you may have taken So Dr. John Smith, PhD, Professor of Behavioral Neurosciences , Department of Psychology teaching a course -- PSYCH 402: Molecular Neurochemistry of Abnormal Psychology, would clearly be a sciences letter even though the professor and the course are from the psychology department. Another example would be Dr. Bob Jones, PhD, Associate Professor of Kinesiology and Applied Physiology, Department of Exercise Science and Sport Studies teaching a course -- EXPHYS 340: physiological responses in sports training. Again, certainly science work even thought it is an Exercise Science course and department. So use your judgement on these.
In sum, getting a letter from a professor who knows you well and can write a critical evaluation of you is much more important than the strict interpretation science