Auto Tangency accelerates the process of sketching a conic curve. When you select this option in the Conic PropertyManager, you set the first and second endpoints for the conic curve as shown below.
The Conic tool in SolidWorks applies tangent relations at each endpoint of the curve and selects the top vertex of the curve as shown below. This option is helpful when you reference existing geometry. If you clear this option, you must define the top vertex of the curve.