Oil grade

Does anyone know what the second rating on engine oil means? 10W-30 for example, 10W means it has thickness rating of 10 when tested at 0 degrees F I believe. The second part means it has the rating of 30 when tested at 212 degrees F if I remember correctly. But Im wondering, engine temperature can go way past 212 degrees, so how does testing and rating oil at 212 degrees mean anything?

Engine water temp can get well past 212, but oil usually doesn't get much past 220 degrees, so wherever they rate the oil is more than sufficient.

oh, ok. Thanks.