Back in the days when I had to file the catalogue cards above the rod (someone had to check they were in the right place before dropping the cards) the concept of a main entry was much more often talked about, and much clearer in my mind. Feel free to correct me in the comments if I’ve gone astray. It was late 1980s when I started doing online cataloguing and the card catalogues were quickly abandoned.
For any book (or other resource) there were a set of catalogue cards. Each card represented an entry point that someone might look for in the catalogue. Only the first three authors (human or corporate) would get an entry, along with up to three subject headings and of course the title and alternate titles. The main entry was the one that would be used to list the book in a bibliography. There were rules to figure out what the main entry should be for a work. A first-named personal author trumped all others. Corporate authors could be a main entry under certain conditions, including if there were no personal authors named, and sometimes a title could even be a main entry if no authors were identified.
The physical capacity of card catalogues and the amount of time for filing put a stopper on the number of entries. The equivalent in online catalogues is every field that gets indexed and listed in order eg. a list of titles, a list of authors, a list of subject headings. Although other fields get indexed for keyword searching there was no equivalent in card catalogues.
Daniel Stuhlman writes about the dying art of filing catalog cards.
And now please enjoy the library catalogue scene from Ghostbusters.