What is "the arm"?

At a recent referee course there was some discussion as to what constitutes “the arm” when considering a knock-on under Law 12.  Some there – with way more experience than I – suggested that it was technically from the elbow down to the wrist. I cannot find a definition of “the arm” in the Laws, but – potentially confusingly – there is reference to “whole arm” when discussing binding.  Any thoughts?
Owain Stone

Owain

Let's not get too hung up on something so simple.   We all know where your arm is, it's from your shoulder to your wrist.  To suggest otherwise is a bit daft!

Ask these 'more experienced' referees to quote the law that says it only applies to the lower arm?

The Rugby Ref