A combined search will retrieve more records than a subject or keyword search alone. For a comprehensive search you will need to combine keyword and subject searches. This is generally considered to be best practice in systematic review search strategies.
A combined search means that you search for your keywords, and also for your subject headings for each concept of your search strategy. Then combine the keyword and subject heading searches with OR. Finally, combine the resulting search results for each concept with AND.
The screenshot below shows a combined search in the Search History of Ovid Medline.