<quote who="Ambrose Li [CCCGT]">
Hmm, I thought that both Dewey and LC have similar functions. But then I'm not a librarian.
Yes, you are right here.
My understanding is that the primary function of both Dewey and LC is to determine the physical locations of a book; they just happen to also indicate a "primary" topic (whatever that means).
The "real" classification (assignment of subject headings) use a list such as Sears (usually used together with Dewey) or the LC subject headings (usually used together with the LC code). Multiple subject headings can be assigned, and in the case of MARC these are placed in the 6XX and 7XX fields. In the case of having multiple topics in a book (which is usually the case), the librarian doing the cataloguing would decide which is the "primary" topic and use that as a basis to assign the call number.
Strictly in librarians terminology, the job of a classifier is to assign the class number to a document using any classification scheme such as DDC, LC, and UDC. The assignment of subject headings (or 'keywords' for IT people) is the job of the cataloguer. Since the cataloguing of a book is done after the book has been classified the subject headings/keywords can be derived from the class number using any method such as "Chain Procedure." The chain procedure of Ranganathan was successfully used in the British National Bibliography (until mid-seventies) to create the subject index. So, I really don't know what you mean by "real" classification :-( Saiful