The "rape" eaten in the north, also called "oil cabbage", has an oily green petiole (the petiole of cabbage is white). Because the leaves are thick and slippery, it is a common side dish in the north (there are many in the south now).
The so-called "rape" in the south refers to oil crops specially used for seed setting and oil extraction. When it was a seedling, few people ate it as a vegetable, but this door made it grow up to collect seeds and extract oil, which is called "vegetable oil".