We have two classification systems, Dewey (DDC) and Library of Congress. (LCC) That in itself is not a problem, and it's fairly easy to explain how they developed in different contexts, always making sure to explain that these systems classify the items in a library, not the world of thought.
What is hard is to try to explain what either of them has to do with the Library of Congress Subject Headings.(LCSH) Many folks assume that LCSH is the entry vocabulary into LCC. Thus if there is a classification code in a record that stands for "vocal music, choruses" that there will be a heading in the record that is "vocal music, choruses," and vice versa. They also assume that the two subject systems (classification and subject headings) have the same structure, which would mean that you can "drill down" from music to vocal music then to choruses in either or both. Nothing could be further from the truth. So it is quite confusing to them when they see a record with a call number that would ostensibly be about "vocal music, choruses" based on the classification, but instead the subject heading is "Cantatas, Secular -- Scores." And they are equally confused when the record has another subject heading ("Funeral music") but only the one classification number.
I can't explain this disconnect between the subject headings and the classification scheme, except to say: that's how it is.
Recently, I was browsing through my beloved copy of the DDC from 1899 that still has both its numeric and alphabetical tabs relating respectively to the classification and the "Relativ Subject Index." The RSI is indeed an index to the classification scheme, and it appears that Dewey originally intended it also as the access to the collection:
"HOW TO USE THIS INDEXFrom this I can only presume that the shelves and the subject catalog were in classification order, and the alphabetical index was the index to that classification. I can only guess at this point, from what he says here, that the subject catalog was in classification order, as is the shelf, but also contained the verbal translation of what the decimal classification numbers meant.
Find the subject desired in its alphabetical place in the index. The number after it is its class number and refers to the place where the topic will be found, in numerical order of class numbers, on the shelves or in the subject catalog."
"Under this class number will be found the resources of the library on the subject desired. Other subjects near the one sought may often be consulted with profit; e.g., Communism is the topic wanted and the index refers to 335.4, but 335, Socialism, and even the inclusive division 330, Political economy, also contain much on this subject. The reverse is equally true; the full material on socialism can only be had by looking at its divisions 335.3, Fourierism, 335.4, Communism, etc. The topics which are thus subdivided are plainly marked in the index by heavy faced type."My copy is #3933, originally owned by the Roger Williams Park Museum in Providence, Rhode Island. The current incarnation of the institution appears to be the Museum of Natural History and Planetarium. My copy has many penciled notes in the area of Zoology (DDC 590), which would fit the natural history nature of the institution. (I don't see any evidence of a current library.) By 1900 the "dictionary catalog" would have taken root, so I don't know if the library would have followed Dewey's instructions for the creation of a classified catalog. But I do wonder how we got from a single system that had an alphabetical index to a classification system to a system with an alphabetical index and two classification systems, but in which the index and the classification have essentially each gone their own ways. This is obviously a gap in my education, which I will gladly rectify if you have suggestions for readings.
Meanwhile, no wonder users are confused.