The Sepia filter imitates sepia toning to produce a black-and-white picture with a warm brown-ish tone.
This slider lets you control how much the effect blends into the original image. 0.000 is no effect at all, 1.000 is completely replacing the image with its sepia-toned version.
This checkbox allows you processing the image in either sRGB gamma-corrected (checked) or linear (unchecked) color space.