Chinese Cabbage, Chinese Broccoli and Chinese Mustard are vague terms that were no doubt coined with the best of intentions.
They were probably originally meant to describe particular variants of Bok Choy, but then one term meant for one variant got applied to seven others, and it’s all such a muddle now that the terms no longer have any utility: they are used interchangeably and seemingly at random by different writers to mean basically any kind of leafy green edible savoury plant coming from Asia.
If a recipe calls for one of these, you’re basically on your own trying to figure out which one the recipe writer intended. Most recipes now also give in brackets an interpretation of the actual Chinese or Asian name that they have in mind.