I am afraid the interviewer was not competent at what he was asking. Such interviewers would ask you
Why are manhole covers round?
And they expect you to say something along the lines of safety. But they would completely disregard other valid answers, such as the ones described here
There are no strictly defined levels as such. Merely some bloggers shared their personal view on the topic. Does it really matter if you are on Level A, Stage 2, or Phase III based on their private classification? No it does not. Shall you be aware of such classification to do CI properly? No, you should not.
The actual number is irrelevant, what is relevant is the description of CI process.