I've seen fonts use these kinds of variations:
and I was curious when to use which. Now I've seen someone say to use SemiBold when using white on a black background. And of course I know to use Bold when trying to emphasize something or Light if its something that needs to be airy, of if the font-size is really huge, and perhaps Black if its something that really needs to stand out.
Also I was curious about the number sizes I've seen also:
100 = ExtraLight
900 = Black
Does 500 = regular?