"Position statement" = collective statement by a group of experts on a topic. "Encompassing field" = group of medical experts who study a larger body of knowledge that includes CFS/ME as a subset.
Like, in physics there is a big field of people who study dark matter, with subfields who specializes in certain (often mutually exclusive) hypotheses about what dark matter is made of, e.g., WIMPs, axions, etc. If I wanted to know whether a new hypothesis about dark matter was plausible, I'd want to hear what all the dark matter specialists thought, not just people who already committed to studying that particular hypothesis.
Like, in physics there is a big field of people who study dark matter, with subfields who specializes in certain (often mutually exclusive) hypotheses about what dark matter is made of, e.g., WIMPs, axions, etc. If I wanted to know whether a new hypothesis about dark matter was plausible, I'd want to hear what all the dark matter specialists thought, not just people who already committed to studying that particular hypothesis.