I think the main difference is in amount of experience including working with other instructors. There is also additional training associated with each upgrade. There are specific topics and techniques that are required for each level. Those are listed here:
Become a Billiard Instructor with the Professional Billiard Instructor Association (PBIA) The PBIA’s mission is to provide quality billiard instruction to individuals at all levels of proficiency, …
I poked around the site but didn't find it where I expected. In retrospect, I see that the "Instruct" link makes sense, but I had basically dismissed that area.