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This can allow you to have complex blending for mowed grass effects for example. To achieve this you would setup UVW which follows the direction of the mowed grass, and modify one of the pattern maps to make use of this. You would then paint vertex alpha in your model only in the areas you wish for the pattern to be shown. At complicated sections where the UVW doesn't really work, paint back in the generic tiling pattern.
Pattern Random Blend
This option allows the pattern blending to be calculated by a random algorithm and ignore the Vertex Alpha. If there is no specific pattern effect on the grass this can be quite useful. It is enabled by default.
Generating Splatter Maps in Substance Designer
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