When does cremini become classified as portabella?

Yes, I know button, cremini and portabella are the same mushroom of different maturity, however, I have not been figure out the classification. Button and cremini area easy to tell by the color, but at what point does cremini become portabella?

Below is from a pack of cremini. It weighs four ounces, so it's right up there with many portabella.

giant cremini or small portabella?