In Enneagram theory, there are three drives, or "instincts" that are key to our survival needs. Some say that they are so deep that they penetrate into our unconscious, dictating our very desires. Each person has a "stacking." This test assesses your dominant instinct. Your second highest result will be your second instinct. For instance, you may be a self-preservation dominant with the sexual instinct as your second result. Then, you would be a sp/sx.
No matter how you view the instincts, it's always fun to take personality tests!
-The test creator (a so/sx)