The Subject Search currently has two uses. Its primary use is to find materials about a topic. This is a critical difference from many of the other searches you do. If you search bernstein leonard as an AUTHOR you will find music written by and performed by Leonard Bernstein.
But if you want to read a book or watch a documentary about Leonard Bernstein, you should search bernstein leonard under the SUBJECT tab.
The terms used in the SUBJECT index are from the list created by the Library of Congress. They follow certain patterns that are helpful to know when you search. Here is one of the subject terms used for a book on French Baroque Opera
4 important subject search hints are illustrated by this subject:
Genre as a topic is usually given in the singular form: Opera not Operas - Sonata not Sonatas
French (German, etc) always means the language French, not anything from the country France; this allows you to distinguish between language and geography: Folk songs, French -- Canada
Musical history is defined by time periods not style terms: 17th century not baroque - 18th century not classical
Subject terms are searched as a phrase and if used must come in the same order: Topic - Place - Time
There is a second, totally unrelated use of the SUBJECT index in the Library Catalog: medium of performance terms. In this Beethoven Clarinet trio, op. 11 example, the subject: Trios (Piano, clarinet, cello) -- Scores and parts can be followed as a link to find all the other pieces for piano clarinet and cello (as a score and parts) in the Library. Shorten the search to just trios piano clarinet cello and you will find everything in the library (scores and recordings) for that combination.