What is the relationship between transparency and opacity in the Principled Shader?

H16 introduced opacity to the Principled Shader but the help docs haven't been updated yet.

Thanks!

Transparency is refraction–like glass or water–opacity is like the ‘alpha’ of the sample currently being shaded, although it is vector alpha, meaning the sample compositing is done per component, and can have a different alpha for red, green, or blue.

I've added some docs to clarify this.

Great, thanks guys.