Why is a song's iCloud status: Matched?

See About iCloud Music Library icons and status - Apple Support. iTunes will always attempt to match a track in your library against those in the store first, only uploading your copy if it fails to find a match.

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Yes the copy in your library remains unchanged. Matching should be done on the basis of an audio fingerprint, but there is scope for mismatching clean and explicit versions of the same song.

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The songs matched or uploaded from the source library remain unchanged in it unless you were to choose to remove the local copy (e.g. right-click > remove download) and then download the cloud version.

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