The reason we didn't define it is that it's incredibly tricky. There are some people with no education and no track record with incredible skills, drive, and professionalism. There are others with long track records who are less skillful in some areas. After trying to write up clear definitions for a while, we realized that we can't do it right without writing fifty pages of legaleze, so we decided not to waste time on it.
In practice, when you have only three levels, most people have a good intuitive feel for where they fall, and it's usually in tune with our evaluation. I agree that this system has its problems, and we'll try to work out the kinks over time. It will be interesting to see how it evolves.
Have you considered defining the ranks based on the job responsibilities rather than the backgrounds of the applicants? e.g., "will need to do X without supervision" / "will need to supervise more junior people" / etc?
Along the sames line, if there is a variation, then write it down as the norms or minimums, at a minimum, they will be able to do 3 out of this 6 things, etc.
In practice, when you have only three levels, most people have a good intuitive feel for where they fall, and it's usually in tune with our evaluation. I agree that this system has its problems, and we'll try to work out the kinks over time. It will be interesting to see how it evolves.