We are in the process of training a SD for my youngest DD.
The basic answer is that each SD may do different things, based on what the person needs. My DD uses a wheelchair - power outside the house and manual (or push) wheelchair inside. She can't get out of her wheelchair by herself. She also can't talk, but does understand.
In my DD's case, here the things we are working on for the dog to do so far:
- picking up things that DD drops
- getting things for DD
- pushing the button that opens a handicapped door (into a store for example)
- getting help for DD if she is in her room and needs another person to come
As he learns those things, we will work with the trainer to think of other tasks the dog can do for DD and he will learn those too.
So, his job will be to help her do things that she can't do herself.