If a field is included in the global search, the prefix can be used to search for specific records only. You can specify a list of prefixes for every field that is included in the search. Lets say you have an address with a company name, a first name and a last name, you could give the following prefixes:
Company: address, company
FirstName: address, name, firstname
LastName: address, name
You could search for "address" and search all the fields, or you could search for "name" and look only in first and last name