Scope of practice defines what a credential qualifies its holder to do. For personal trainers, the certifying bodies publish these boundaries themselves.
Published scope-of-practice statements from the major certifiers describe the trainer's role as designing and coaching exercise programmes for apparently healthy people, conducting basic fitness assessments, and providing general information consistent with public dietary guidelines. The same documents state what falls outside the role: diagnosing or treating injury or disease (licensed medicine and physiotherapy), prescribing diets or supplements for medical conditions (registered dietitians), and counselling for mental-health conditions. They direct trainers to refer clients to licensed professionals in those situations — which is also the practical test a consumer can apply to any online coach's claims.