this is a difficult question to answer. It is because the N Chinese reference of 白菜 is different than S Chinese 白菜. The Kimchi type of 白菜 is called napa cabbage. I think the N Chinese call napa as 大白菜. Am I right? While the S Chinese called 白菜 usually referring as bok choy. Napa and bok choy all belongs to cabbage family but they look very different than each other. Bok coy looks very different than the European cabbage. That's why N American supermarkets commonly called 白菜 as bok choy. Bok choy - one can easily see that is a straight phonetic translation of Cantonese to English. Another interesting observation - usually bok choy stalks are white, yet there are bok choy with green stalks.Did I confuse you enough?