Need help understanding 16QAM constellation diagram Is the identification of each point on the constellation diagram arbitrary as long as the sender and receiver know the mapping?

I have seen some diagrams which order in an outward spiral, while others go left to right. Does it matter?
Yes and no.

The mapping is arbitrary as long as the receiver correctly determines which constellation point a symbol is.  If the receiver makes a mistake, though, it is most likely going to pick a "neighbor" constellation point (i.e. a constellation point that is only one spot away).  It is highly unlikely that a correctly implemented receiver will pick a constellation point far away from the transmitted point.  Communication systems try to mitigate the impact of the "neighbor" mistakes by minimizing the Hamming distance between neighbors.  A map where all of the nearest neighbors have a Hamming distance of 1 is called a Gray map.  If you use a Gray map almost all of the symbol errors will only result in one bit error.