The shading rejection run a network of convolutions on the GPU all in single 32x32 without roundtrip to main video memory. However chaining many convlutions in this tiles means that some convolutions on the edge arround are becoming corrupted and therefor need to overlap the tile by couple of padding to hide it. Higher means less prones to tiling artifacts, but performance loss.
Usage
r.TSR.ShadingRejection.TileOverscan [value]
Set via the console (~) or in DefaultEngine.ini / DefaultGame.ini.
