In the legacy Mapbox Studio Classic product, a classic style contained CartoCSS stylesheets, thin metadata (name, description, attribution, etc.), and a reference to a source. After you uploaded a classic style to Mapbox, it appeared on your Classic styles page.
Creating a new map? Mapbox Studio is the modern alternative to Mapbox Studio Classic. If you're creating a new map today, you should use Mapbox Studio instead of Mapbox Studio Classic.