It's nothing I made up, I have been doing that for years now, ever since I returned from the 1st Foreign Instructors Training Qualification Training Course at the Kukkiwon's World Taekwondo Academy in 1998.
It was then debated at length in the early days of internet Taekwondo debates. I received written clarification for both Kukkiwon and WTF, and the main editor of the WTF, Kukkiwon and KTA websites, GM LEE Kyong Myung (co author of the Modern History Book). He was the reviewer of the English Edition of the Kukkiwon Textbook, Chairman of the WTF Education Committee and main writer of the WTF Poomsae Judges Textbook.
I also asked this question of GM's PARK Hae Man and KIM Soon Bae, all writers and editors of the Kukkiwon Textbook. Even after all this, it is still a hot potato, but that is why I enjoy it so much! It certainly rocks the boat of so-called tradition that has been set in place in foreign Taekwondo schools. Reason? Before 1998, on Korean Taekwondoin could take the instructor course, so all kinds of weird rules popped up to fill in the "instructor" title gap.
Here is what we have to consider: Yudanja is 1st to 5th Dan. I think we have known this for a long time, but not really paid attention to it. Kodanja is from 6th to 9th Dan. We have known this too, for a long time. Why the separation?
We have also known, well, since 1998, that Sabum was not just some title given out at 4th, 5th or whatever dan, that it is an actual designation by Kukkiwon. It means instructor, and according to Kukkiwon, the only way to receive this qualification is to #1. attend the "jidoja yunsuwon" or leadership academy course held by the "World Taekwondo Academy" at Kukkiwon (now in some other nations as well, like USTC in the USA), which is held for us foreigners once a year in Korea around July. Once you have successfully completed that course, and received your "graduation diploma", you can now use that diploma and apply to take the "Sabum Gye Gyuk Jung" exam (only if you hold 4th Dan Kukkiwon and above). If you pass that test, you will gain a certification from Kukkiwon that states that you are a qualified "Taekwondo Sabum", or Taekwondo Instructor. Their are 3 classes of Sabum. 3rd, 2nd and 1st.
Of course in any dojang, anything goes!