Hilchot Shabbat 7:4-5 (The difference between an “av” and a “toladah”)

Yesterday the Rambam listed the 39 avot melachot – that is, the 39 prime categories of forbidden labor. In addition to the prime categories, there are numerous subcategories for which one is equally liable. What’s the difference between an “av” – prime category – and a “toladah” – subcategory?