Explain Milling extraction rate

The chemical composition of the flour depends on the milling extraction rate.  Increasing the rate of flour extraction decreases the proportion of starch and increases the amount of kernel-coating constituents such as minerals, vitamins and crude fibre. Comparing products of the same extraction rate, rye  flour contains higher proportions of both minerals and vitamins than wheat flour.  In case of some B-vitamins, such as niacin, this difference is well-balanced by the higher concentrations in wheat in comparison to rye kernels.