The three instincts were originally used with the personality types to create three subtypes or variations of each type: self-preservation (sp), sexual (sx), and social (so) subtypes. A more recent approach to the instincts determines a preferential order to all three instincts rather than simply looking at one instinct. This preferential or stacking approach can also be used independently of the types where the instinctual stacking is used without reference to the dominant personality type.
More details about the instincts can be found in the Enneagram User Guide series book Personality Variations. This book discusses the instincts and the various uses as applied to Enneagram type and independent of Enneagram type. Check the home page for the latest information about the availability of this book, other books, and related projects.