I'd classify it as having an occupation which requires a specific qualification at degree level or above (usually postgraduate). Also there would be a professional governing body.
So doctors, vets, dentists, accountants, engineers, midwives, lawyers, architects, social workers, teachers, nurses, university professor, post doc scientist, senior civil servants.
Ones I'm not so sure about: journalists, bankers, politicians, senior police officers, physiotherapists, radiographers, occupational therapists, 'managers', tv/radio/film directors/producers/editors, book publishers, museum curators, auctioneers, firefighters, prison guards, hairdressers, health & safety inspector, tax inspector, charity CEOs.