I think you are confusing a CROP - which is done for composition reasons and a 100% CROP which is usually done for education / equipment testing - as I said above, the primary purpose of posting a 100% crop is to show sharpness (or lack of sharpness), or to draw attention to a certain area of a shot. It is not usually meant to be a final image for printing.
Specifically, a 100% crop is when you open your photo in Photoshop, and without resizing the shot, then using the rectangle selection tool, select an area, then crop away every thing other than the selection. For the purpose of posting here, you want the selection to have a longest dimension of no more than 800 pixels.
The resulting image is a 100% crop. Because 1) it is at 100% of the native shot, and 2) it is a cropped portion of a larger shot.
In THIS POST, the lower shot is a 100% crop.