General Practice.
If you are wanting to be a General Practitioner then you will be treating lots of different conditions and will looking at finding patients in your locality. The approach to increasing awareness of your business is aimed to a smaller area and a wider audience. Of course you always need to treat the individual and even if you specialise, you will always need to understand and be able to treat the whole range of conditions as people are individuals and will have a different combination of issues and problems for you to treat.
Treating Specific Conditions.
If you intend to specialise your treatment to a set of specific conditions, for example sports nutrition, then you would advertise in specific magazines, your audience would be smaller and specific but the area could be wider.
Starting General then moving to Specific.
Perhaps the easiest option is to start general and then move into a specialism. You have to build up a reputation as a specialist and that can be much harder. You may also think about research and publishing papers to build up your reputation. Alternatively, you could think about writing a book on your subject and increase the knowledge of your specialism.