The default font is a trimmed-down (Latin only) version of cc.yal.6w4. If you don’t need the rest of 400-something glyphs in the font, you can use it for whatever - it’s there as an example of a proportional font that looks good enough.
At the time it seemed like $10 is a good price for a font that covers a variety of languages and took long to refine, but people now offer dozens of fonts for less than this, and many fonts are free as such, so most of my fonts now sell 0 or 1 copies per year.