Here's the process. They have a sensor that is fixed into a block. This is connected to a data acquisition device, which in turn sends the data to a computer. The sensor has adapters to fit 1/4", 3/8" and 1/2" square drives. They put the torque wrench in the adapter, adjust it to the lowest setting and apply torque until it clicks. They do this a few times, move to the middle setting, repeat the process and then move to the highest setting and repeat the process. If the torque wrench is more than "X" percent away from the actual torque that it took to make it click, it is either sent out for repair (back to the mfg.) or scrapped and replaced, depending on the value of the wrench vs. a new price. They couldn't set a number on the percentage, because it will vary depending on the application. But, with some arm twisting, he said within 2% to 3% is what they commonly looked for.
Hope this is what you were looking for.