The upper position refers to a more general statement, and the lower position refers to a more specific statement. In fact, these are all relative.
For example, if you focus on the types of screws, the lower ones are long screws, short screws, and square screws. At this time, your upper concept can be generally written as screws. But if you want to emphasize the fixing function of the screw, you can turn it into a fixing device or a fixing component.