a) yes you still have to do an assessment to get in to an area unit
b) the best being at specific units would require some sort of selective posting of trainees if you believe it's by selection, which isn't supposed to happen, or by becoming the best by working in the busier places, which may or may not happen depending on the individual. You never know if someone could do the 'busiest and best' unless they try it, posting does not necessarily reflect ability.