It is an organization made up of individual programs that train dogs. You will have to contact each organization individually to get more information. The short answer is Golden Retrievers and Labrador Retrievers. There are exceptions of course. Working dogs are easy to train but tend to be protective.
Field dogs tend to more interested in their environment than people.
Small dogs can't pick up large objects or pull wheelchairs. Large dogs are hard to put under a table in a restaurant or out of the way on a bus or plane. A good Service Dog is not protective, is people oriented, is not overly active, and is confident but not dominate or submissive. Since most Hearing Dogs are rescued from shelters, most of them are mixed breeds. They come in all different sizes, shapes, and colors.