The 50 pts extra. Think of it this way. I have X and y amount of units in my 20k army list. If we play 2000pts, I can only fit 2x and 3Y in that list. Probably leaving me short by like say 50pts of putting in a Z unit or another X. That 50pts just raises the celing for you and can allow for fitting more things in. you won't be able to bring everything you want and that's kind of the point. this bonus helps with that.