From Diary of a Screenwrite:
Stanley Kubrick insisted that a feature film can be constructed from six to eight ‘non-submersible units’. A non-submersible unit is a fundamental story sequence where all the non-essential elements have been stripped away. These units would be so robust and compelling that they would, by themselves, be able to keep the viewer interested. They would contain only what is necessary for the storyline. And when joined together they would form a greater narrative.
The way Kubrick reduced 2001: A Space Odyssey to its most important elements was indicative of his emerging method of telling stories. Over the years, Kubrick had adapted many books into films. By the time he came to conceive of 2001: A Space Odyssey he realised that all he needed – as he later told science-fiction writer Brian Aldiss – are six or eight ‘non-submersible units’: basic story points that cannot be reduced any further. When the story points are linked together they form a narrative that will contain a balanced mix of all the themes, images and characters.