What is the difference between a 40m javelin rating and a 50m javelin rating?

I saw a video on YouTube where the fellow explained that the 40m javelin was not as stiff as the 50m. A more flexible javelin is preferred by beginners as it absorbs some of the stress better and relieves that from the shoulder. As a thrower gets in better shape and throws farther, the stiffer 50m javelin is preferred as it flies better and, thus, farther.