What is the meaning of Thành_phố_thuộc_thành_phố_trực_thuộc_trung_ương?

a so-called "municipal city" or "city within city", a novel designation for former districts in a thành phố trực thuộc trung ương, whose size and significance grew large enough to be considered a đô thị, currently held only by Thủ Đức

Source: wiktionary.org