This density refers not to a component itself, but to soldering. There is a standard named IPC-7351 (7351A, 7351B) "Generic Requirements for Surface Mount Design and Land Pattern Standard" that defines pads size for the same package. In short low density is the biggest pads size, medium density - medium pads size, high density - the lowest pads size. The type you need depends on your requirements, i.e. for hand soldering it's better to select low density. Although the copy is not free, you can google older versions (pure IPC-7351) and find general info about land patterns, producibility levels etc.