Pre-release and unused content
From the Super Mario Wiki
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Pre-release and unused content are components of a video game that, for whatever reason, are removed or altered before that game is released. Known info about pre-release elements usually comes from one of two sources. The first is information released about a game by its creators before the game is complete. The second is data found within the data of a game that serves no purpose but could have been implemented into the game at one point.
Pre-release and unused content definitions:
Pre-release elements should not be confused with unreleased media, entire games which never see release.
To some extent, this can apply to television shows and movies. For example, there were some early depictions of characters and deleted scenes found for the Super Mario Bros. film, which can be often found in the special features section of DVD releases.
The full list of pre-release content can be found here.