Actually, there are 3 distinct levels of SDs. The one is a regular SD, who helps a disabled person with distinct sets of commands...and they are usually trained to about 25 commands if one reaches the program they are seeking to get a dog from. The commands increase, or are tweaked, for each individual. Not every disabled person is in a wheelchair. Some SDs help balance their owners who may have irregular gaits, or are somewhat weak when they walk. Some SDs are for those whose disability is more of a need for a calm dog because they might have cerebral palsy, have had a head trauma, or other disabilities that makes the SD a family SD, as the whole family will be trained to use the service dog for the person who is disabled. SDs are trained from the time they are puppies, and have a rigorous training schedule when they arrive at the organization they were bred for. This training may take as long as 2 years to complete. They are trained in much the same way a Seeing Eye Dog is trained.
The PSD, or psychiatric service dog, must have at least three commands that they do regularly for the individual they work for. PSDs are seen as SDs, but highly individualized for the individual. They are able to go out and about with the owner wherever the owner goes. PSDs can be trained to recognize different states so that they can help their owner come back to a more centered place. They will sometimes on their own pick up cues from the individual and will find a way to help that individual, by snuggling close to them, or laying their head on the person's lap, or will try licking the individual until that person is aware of his/her surroundings again. As with SDs, the dogs must be calm and sociable, not prone to barking or straining to get to something it sees laying on the floor, unless they have been specifically trained for that command. They are quiet and draw no attention to themselves, much in the same manner as SDs appear invisible to others around them.
The ESA, or emotional service assistant (dog or cat or other species of animal), is not allowed out and about with the owner at all times, but rather is a part of the family structure, and is used to calm the person down in time of unrest, or give assurance to the owner that the animal is there when there is no other person around. The animal may accompany the family in the car, but the animal does not have the license to accompany the owner into a store, restaurant, or other common place. They are not seen as an SD, or PSD, in any way, shape or form. Getting a PSD trained can be done with a local trainer, or may be owner trained. We have a PSD in training right now, and chose a small dog because we didn't need a large dog to help us. She is doing well in her training, and will be trained within a year. The benefits of getting the dog locally trained, or even owner trained, is that you can tweak the commands for your own use. And then can take the test out in public so that you can be out in public as a PSD. Being physically disabled, I could have a regular SD or a PSD, but since my pdoc wrote the letter to whom it may concern, I chose to have a PSD. It is in my official file as necessary to my needs that I have a PSD.