When one of your classes fills to capacity, you'll see a "Sold out" badge and a small celebration animation the first time it loads on your device.
When confirmed bookings for one of your class sessions reach its capacity. The class card in Today's Classes gets a "Sold out" badge.
The first time you load that day's dashboard with that class showing as full, a small floral burst plays once. The badge stays even after you've seen it — only the animation is one-shot.