I have heard that they stopped approving (sometimes they go through, sometimes they are kicked back) ranges of lengths such as 5"-16". So, folks just started listing all the common lengths they may want eventually (7", 10.5", 11.5", 14.5") etc. as well as any calibers (.22lr, 9mm, .45 acp, 5.56, 7.62) they may want, so a change letter doesn't have to be sent every time you want to swap an upper.