These are releases that aren't at the level of singles getting music videos on the main MixmstrStel YouTube page or on my main music pages for whatever reason and may or may not be released on the Mastered! series.
Source quality is usually a factor in labelling these as demos, among other factors. Demos also serve as a way to showcase ideas, especially with (newer) songs that may not have the best quality vocal or instrumental.
For this demo, Kate Bush's vocals from Running Up That Hill are put on top of Ellie Goulding's Something In The Way You Move.
Songs and sources used:
Ellie Goulding - Something In The Way You Move (Instrumental)
Kate Bush - Running Up That Hill (DIY acapella extraction)