What exactly are Opus numbers and how can I apply them as a cataloging system for my own music?

What exactly are Opus numbers, and how can I apply them as a cataloging system for my own compositions?

Opus numbers are numbers based on when the pieces are published. Pieces published together or in quick succession, that's a single opus. Pieces published with a gap, that's 2 different opuses.

This sometimes but doesn't always correlate with the composition order. An example of a piece where the composition order and opus numbers are opposite is Chopin's Nocturne in E minor.

By that high opus number, opus 72, you might think that it is a very late composition. But it is actually from Chopin's early style, a similar style to that the first 2 opuses of his nocturnes are in, it just wasn't published until after his death, thus the high opus number.

Beethoven's opuses more closely align with compositional order, but he also has a lot of WoO works. WoO means Without Opus. I've rarely ever used Opus numbers personally, so you could say most of my works are WoO.

Got it. I had heard from some other people that composers apply them themselves when they are written. But it seems to make more sense that it's based around when they're published. Thank you.