Don't quote me on this but I always thought or led to believe that this socket (same with toyota) was used with the dealer tool which when plugged in to this socket allowed more functions to be carried out.

functions such as timing adjustment, idle speed adjustment etc and the 16pin socket was for complience allowing engine dtc's etc to be read.

Isn't this also similar to BMW of around 2000 year where they have both 20 pin and 16pin diagnostic ports. ABS codes can be read via 20 pin but not 16 pin.