Ensures that all deforming vertex groups add up to one while painting. When this option is turned off, then all weights of a point can have any value between 0 and 1. However, when vertex groups are used as deform groups for character animation then Blender always interprets the weight values relative to each other. That is, Blender always does a normalization over all deform bones. Hence in practice it is not necessary to maintain a strict normalization and further normalizing weights should not affect animation at all.