When this icon is selected, the data created will be compatible to PostScript Level 2. One advantage of PostScript Level 2 over Level 1 is, among others, that it can handle CMYK colours and that the page formats can be set more precisely.
We recommend basically that you use at least Level 2 format output to avoid colour and positioning problems. But as not all PostScript interpreters are Level 2 compatible (e.g. Calipso v1.x), you can switch off this option here.