I think these questions have been asked, but not as explicitly as I want to put them, so here goes:
Commonly it is said that you need two science professor LORs, and one non-science professor LOR, at a minimum.
1. Do these professors have to have taught courses that you took?
2. Is "science" here specific to biology, chemistry, and physics, or is it broad? Would earth science or computer science professor's letter (I'm aware they both have science in the name) count as a science or a nonscience LOR?
3. Looking at the excel spreadsheet, the vast majority of schools do not explicitly require a non-science professor LOR. They seem to mostly require 1-2 science professors and leave the third one open. So why is it stressed so highly to get a non-science LOR?