Keywords may appear anywhere (title, abstract, full-text, even the author's name!) in a record. Searching with keywords also depends on the search algorithm being used by the database. It can be a good way to find results the rely on a particular spelling, combination of terms, or of a new term. The search algorithms used in most databases are good enough to surface what you are looking for.
Subject Headings are a way of using controlled vocabulary to apply specific language that describes the content of the article to ensure that all the entries related to a particular topic are tagged with the same heading. They are different from keywords because they relate to the content of the article and can give precision to searching.
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Use drop-down to add a MeSH Term