Actually, it's all as clear as mud, like anything to do with music.
(Mud is therapeutic if it comes from the right source, so I've been told......though the one from the Dead Sea didn't turn the clock back 100 years on my skin
Opus is Latin for work (or art). Composers or their publishers may assign opus numbers to their compositions, whether a one-minute mazurka or a ninety-minute five-movement symphony. And there may be more than one piece grouped into each opus (often because they were published together), therefore Sonata in C# minor, Op.27 No.2 (not 'Moonlight' please).
But given composers' inconsistent or inexistant assignment of opus numbers, especially during the Baroque (1600–1750) and the Classical (1750–1827) eras, musicologists have developed other catalogue-number systems; among them the Bach-Werke-Verzeichnis (BWV number), and the Köchel-Verzeichnis (K- and KV-numbers) which enumerate the works of Johann Sebastian Bach and Wolfgang Amadeus Mozart, respectively.
D.Scarlatti's sonatas have been catalogued by a few people. For many years, Scarlatti's keyboard sonatas were conventionally identified by their Longo (L) numbers, but these were later superseded by those found in Ralph Kirkpatrick's catalogue. (Kk numbers - don't use K to avoid confusion with Köchel for Mozart).
However, some works are so famous that no-one (except me - rarely) bothers with opus numbers or musicologists' assigned numbers to identify them:
Thus J.S.Bach's Goldberg Variations, BWV 988, but since The Silence of the Lambs
, everyone knows all about Count Hermann Karl von Keyserling's insomnia, so we don't bother with the BWV number generally. We just call it "The Goldberg"
As for Handel's Messiah, whoever bothers with the HWV (Händel-Werke-Verzeichnis) number?? Everyone who's anyone has sung it in the bath.
And Mozart's Symphony No.41 in C, K551 "Jupiter", but as everyone knows all about Jupiter (and I don't mean the one in the Solar System), we don't bother to use the K number, nor the numbering of the symphony, and just call it the Jupiter Symphony.
Similarly for Beethoven's Pastoral Symphony (Symphony No.6 in F, Op.68).....but don't confuse it with his 'Pastoral' Sonata, Op.28
Schubert is a funny case - only a small minority of Schubert's works were published in his lifetime. Not all published works were allocated opus numbers, and the sequence of opus numbers that were allocated bore little relationship to the order in which the works were either composed or published. Thus came Otto Erich Deutsch to clear up all the confusion, especially with all those posthumous opus numbers. We should stop using opus numbers with Schubert because they are utterly confusing, and stick to D numbers, if it's required for identification.
Here is his paen to music, An die Musik
, D.547 (disregard Op.88 No.4):https://www.youtube.com/watch?v=3TDEyW9JLuc
......but again, everyone who's anyone knows it, so we don't bother with numbers. Just identify it by the name of the song. (Just like pop singers
Cheers! To Music!!