From X-Plane SDK
The slip range is +/- 8 degrees. It appears to be based on the angle in degrees of the effective G force, e.g. when you are in a slip your effective force is not going along the longitudinal axis of the plane - a side force deflects it. More than an 8 degree effective offset of this angle causes the max slip. (Math: this is basically atan2 of the side force vs the normal force.).