Many people have trouble retaining length because their ends are rubbing against their clothes, their ends are exposed to environment (manipulation, wind, sun), over-processed hair, and/or use of excessive heat. This can cause dryness and lead to breakage, especially if the ends are not properly moisturized. Your ends must be cared for since they are the oldest part of the hair strand and has went through the most wear and tear.
By wearing the hair up, wearing protective styles, and keeping the ends well-moisturized, this prevent excessive split ends and dryness which leads to breakage. This helps you retain all your length. Have you ever seen some girls go get a touch up, and walk out with the same length hair as they walked in with? You would assume that if they relaxed their hair (had new growth) it would be longer, correct? Assuming that they did not get a trim/cut, they are "stuck" at the same length because their hair is breaking off at the ends just as fast as it is growing.
Sorry for the long response, I tend to be a little long-winded. But this is what retaining length means to me...keeping all the hair you grow on your head! HTH!
ETA: Shedding, IMO, has no effect on the growth of your hair, only on the thickness/fullness of the hair.
Breakage affects length.
Shedding affects thickness.