Metadata is how the world finds you
Your artist name is a key, not a label. Spelled one way it joins your catalogue, your followers and your listener history. Spelled a different way — an extra space, a changed capital, "The" moved to the end — it creates a second artist who happens to have your name and none of your audience. This is the most expensive small mistake in independent releasing, and it is entirely preventable by writing your artist name down once and never typing it from memory again.
Featured artists never go in the title
The rule is explicit on every major store: featured and collaborating artists must not appear in the track or product title. They go in their own artist field, at track level, with the role set to featured. Put them in the title and three things happen — the release can be rejected, the featured artist gets no credit on their own profile, and the algorithmic link between the two artists is never made. That last one is the real cost, because that link is how their listeners find you.
Version information has its own field
"Radio Edit", "2011 Remaster", "Live at Brixton, 2019" — all of these belong in the version field, without brackets, one version per field. Not in the title. And an original version carries no version information at all, which means "Original Mix" is not a version; it is a rejection. This trips up electronic producers constantly, because the convention in that scene is the opposite of the convention the stores enforce.
What stores treat as a search term
Anything describing the music rather than naming it. "Sleep Music", "Study Beats", "Best Of", "Greatest Hits", "Music for Concentration" in an artist field are rejected as attempts to capture search traffic. In a title, cover-version disclaimers like "Made Famous By", "In the Style of", "Originally Performed By" and "Tribute to" are rejected for the same reason. So are dates, instruments, affiliations and label names in the artist field.
Capitalisation, emoji and the small things
Titles in English are capitalised on the first and last word, with prepositions of four letters or fewer left lowercase unless they are part of a verb phrase. All capitals is normalised unless it is genuinely your styling. Emoji are rejected. Double spaces create a different string, and therefore sometimes a different artist. None of these will ruin a career on their own, and all of them together are the difference between a catalogue that looks professional and one that looks improvised.