I have a German Shepard Dog (GSD) and while I haven't put him through Schutzhund training both of his parents are Schutzhund 3 and the guy I bought him from trains the vast majority of the police dogs in the region. I bring mine out once a month or so, more for him to play than train, and I am pretty familiar with the training.
They don't necessarily train the dog to bite anywhere. Generally when a man is standing they go for the arm because in training the guy in the big red suit puts his arm up (as do most people when being attacked by a beast of a dog) which allows the dog to bring the man to the ground which is the goal. Once on the ground, dogs generally go for the neck. Why, I assume it's because instinct over the centuries taught them that is the best way to kill their prey. Animals, including humans, have instincts too which generally involve protecting vital areas like the neck.
Bottom line is from what I have seen of both highly trained dogs and dogs being trained is that if you are on the ground the general head area is where they are going to bite. The biggest issue that I see with police dogs is the handler not being nearly as trained as the dog.