only used by itself (no ONyomi)
貝 (’sea shell’) is a classic SOLO kanji. If you look it up in your dictionary, you’ll find like 20 jukugo: 貝中毒、貝割れダイコン, 貝塚 ,貝形模様の飾り, 貝採集, 貝柱 、 貝殻, etc. But unless you’re a marine biologist, you’ll never use any of them! Fortunately, the jukugo all mean ’sea shell’, so 貝 is not a COCK – there’s no exceptions to the meaning or ponunciation.
Since 貝 is used by itself, so usually you can get away with not learning the ONyomi. This is why it helps you to learn the SOLO tag.
Also, even if a kanji is used often in compounds, BUT IS PRONOUNCED WITH ITS KUNYOMI, it’ll get a SOLO tag – since you still don’t need to learn the ON!