There are a range of Professional Doctorates available, for example, Doctorate in Nursing or Doctorate in Education, and different institutions offer other pathways such as doctorates in allied health subjects, and in health and social care more generally.
Different institutions have varying requirements but the following are a guide: you must be a registered professional in the field of study, have been working in that profession for at least three years, and are currently working as a professional in that field as your study topic should be linked to your professional practice.
The professional doctorate includes a more formally taught component, usually the first two years, during which you will study accredited modules focused on research and the professional context. You may be able to start the professional doctorate without a definite study topic and you can then develop your project proposal during the taught part, which is followed by your independent research component - usually a further three years.
The thesis you must submit will be shorter (e.g. 50-60,000 words) as you have already completed assessed work in the taught component. However the requirement to have developed new knowledge still stands.