Style Breakdown (also called Style Mix) gives you an overview of a portfolio's stock ownership/coverage in each of the 9 boxes of the Morningstar Style Box. The numbers are stated as a % of market value.
boxi = sum(mvi)
boxi = style % for ith box
mvi = market value for ith stock
So, for one of the 9 boxes, all of the market values %'s of the stocks that fall in that particular box are summed to get the box's style %.
9 Style Boxes: Large Value, Large Blend, Large Growth, Medium Value, Medium Blend, Medium Growth, Small Value, Small Blend, Small Growth
The boxes are also combined into the following groups: Large, Medium, Small, Value, Core, Growth