The Enneagram instincts are self-preservation, sexual (also called one-to-one and intimate), and social. Although rarely mentioned, the instincts are used with the Enneagram types in two quite different ways: the subtype approach and the stacking approach.
The subtype approach is the original of the two. It's used to distinguish three variations or subtypes for each Enneagram type: the self-preservation (sp) subtype, the sexual (sx) subtype, and the social (so) subtype. The nine Enneagram types are subsequently expanded to twenty-seven subtypes (i.e., three subtypes for each of the nine types).
With the subtype approach, you either determine which of the nine types applies first then which of the three subtypes for that type or you simply determine which of the twenty-seven subtypes applies.
The stacking approach is more recent. It doesn't simply try to determine which one of the instincts is dominant for a type but looks at the preferential ordering of all three instincts. Instead of simply identifying with self-preservation (sp), the individual would have self-preservation as the first instinct and either the sexual instinct second (sp/sx) or the social instinct second (sp/so). The third instinct is implied by its omission (e.g., sp/sx has so third while sp/so has sx third).
The obvious difference between the two approaches is that the subtype only designates the primary instinct while the stacking approach designates the preferential order for all three instincts. However, the differences are much more than simply that.
With the subtype approach, instinct is not viewed independently of type. Someone who identifies as sp 1 and someone identifying as sp 2 have nothing in common. A dominant self-preservation (sp) instinct has only to do with determining variations of type. In other words, the instinct is simply a way of creating three different subtypes and not a way of exploring similarities between types.
In the stacking approach, instinct is independent of type. You can actually use it without referring to Enneagram type. A person identifying as sp/sx 1 does have something in common with someone identifying as sp/so 2. They both have a primary preference for the self-preservation (sp) instinct. You could say that regardless of Enneagram type two people identifying with the same stacking would have a lot in common (e.g., sp/sx 1 and sp/sx 2).
People not realizing that there are two very different approaches to using the instincts with the Enneagram types tend to combine the two approaches and not even know it. This can be a problem when an author writes about the instincts using the subtype approach but the individual uses those subtype descriptions with the stacking approach. They don't really go together because the subtype approach looks at type then instinct to determine variations of type while the stacking approach looks at instinctual preference and may or may not even look at how that affects type. Another way to say it is that a subtype description may not make a good stacking description.