The form for adding new search option to the items search has pull downs for setting the field and subfield to be searched. The lists are a bit "too optimistic" und not very convenient to work with - for example: the subfield list is missing capital letters as often used for additional item fields. Setting the capital letter with SQL, the feature works nicely. And the field list has 999 entries... :)
*** This bug has been marked as a duplicate of bug 20689 ***