​[...]​

​I assume the concept of a main interpreter is inherited from the previous levels of support in the C API, but what exactly is the significance of being "the main interpreter"? Instead, could they just all be subinterpreters of the same Python process (or whatever the right wording would be)?​

It might also be helpful if the PEP had a short description of what are considered subinterpreters and how they differ from threads of the same interpreter [*]. Currently, the PEP seems to rely heavily on knowledge of the previously available concepts. However, as this would be a new module, I don't think there's any need to blindly copy the previous design, regardless of how well the design may have served its purpose at the time.

-- Koos