Generally, you use the pretext/pretext script by supplying a component (-c) and a format (-f), along with your PreTeXt source as the very last argument. The component may be some limited subset of your document that needs specialized treatment, such as an image described by the Asymptote language (-c asy). Or it could be the entire document (-c all) being converted to some format as a final output form. So -c asy -f svg would produce Asymptote images in SVG files, while -c all -f latex would produce the entire document as a file.