I’m the most confused by subject access entries and terms while cataloging. So far, I’ve mostly dealt with 610s and 650s, which are corporate names and topical terms. The part that’s confusing is figuring out the terms that will describe the document your cataloging, that also follows LOC’s authorized terms. Okay, the real confusing parts are the subdivisions. I’m sure it’ll get easier over time once I get more experience, but until then Classification Web will be my bestie.