ok so I'm a PT in med school looking to possibly become a physiatrist. Taus has it right regarding physiatrist vs PT.
regarding PT vs PTA: a PT is the only one who can evaluate a pt, make changes to the care plan, assess the progress of the pt, and make recommendations to the physician supervising the plan.
the PTA plays an important part in being able to take care of the pts along the way by following the PT's plan, treating the pt, and updating the PT on changes and recommends different treatments for the PT to consider.
So the way this usually works in the acute care setting is that a PT will spend a good portion of their day seeing first time pts for their evaluations and making preliminary recommendations to the physician about where they expect the pt to end up after therapy. the rest of their day will be with pts they have seen before doing treatments.
the PTA will spend their day addressing the pt's needs based on the PT's assessment and recommendations for treatment program. all of the PTA's notes must be signed by the PT.
finally the PT will complete the discharge for the pt which is essentially another type of evaluation that documents the progress the pt has accomplished during their therapy.
this is the quick and dirty version and is simplified but should answer the basics of your question.
J