Mounting Screws

Hello, so while this may be somewhat of a "duh" question, this has been bothering me for a while now. How can you tell the difference between screws that are intended for the motherboard and screws that are intended for things such as fans and optical drives? feedback would be greatly appreciated.

Mother board screws have a round disk, kind of like a washer at the top. Fan screws are large with wide threads so they can self thread in to plastic, the ones for graphics cards and stuff like that have a hex shaped end. And hard drive and ODD screws are whichever ones fit :P (I can never remember, guess i've been using toolless for too long ;))

So motherboard screws are normally round, and then screws for things like fans and GPUs have a Hex-shaped screw. And I'm still kinda confused about the ones for ODDs. I've seen cases like the Corsair 200R come with separate bags for motherboard screws, ODD screws, and etc...