Share 'Screw sizes?'

Just trying to get ready to start to make some items after the new year and was looking at getting a few different screw sizes. I noticed some listed by length, coarse or fine, then #8, #7, #2. I understand the length as difined by Kreg. To use course on soft woods and fine on hardwoods. Is the # size the square drive size or is this in reference to the diameter of the screw or what? I hope this…