In this palette, the following settings can be adjusted:
1. Focusing Distance: by default, it is set to infinity.
2. Distortion: Correction of the geometry distortions caused by the lens.
3. Chromatic Aberration: this is one of the most interresting part (the chromatism being strong with a digital camera). It contains the Purple Fringing and the Lateral Chromatic Aberration. The Chromatic Aberration are adjustable manually. It's up to you to optimize this setting working well by default.
4. Vignetting: it is known for your lens and is very efficiently correct without using a flat (a flat is hard to make with a wide angle lens!).

Let's detail those options.

The distortion correction allow to "flatten" the picture.

On the right: a RAW file before and after the distortion processing.
Canon 350D + Sigma 18-50 at 18 mm open at 2.8 on a tripod. 30 s at 800 ASA.

The chromatic aberration correction allows to reduce, possibly removes, the lateral chromatism and the purple fringing.

On the right: RAW file (a crop at 200 % around the Orion's Belt) before and after the chromatism processing.
Canon 350D + Sigma 18-50 at 18 mm open at 2.8 on a tripod. 30 s at 800 ASA.

The vignetting correction lighten very efficiently the photo corners.

On the right: RAW file before and after the vignetting processing.
Canon 350D + Sigma 18-50 at 18 mm open at 2.8 on a tripod. 30 s at 800 ASA.