Transmission and Opacity are pretty similar in Arnold compared with other render engines. However, there are a few differences.
Arnold has two different ways of calculating Transmission and Opacity. They are different ray types and thus have different controls in the Standard Surface shader as well as in the render options.
These two ways of calculating Transmission have different purposes, they can be used together, but most of the time you'll want to use either one or the other.
More information can be found here.
As this is a frequently asked question we have provided both the question and answer in this thread.
Lee Griggs
Arnold rendering specialist
AUTODESK
