public Vector2 surfaceUp { readonly get; set; }

Both _surfaceSlideAngle and _gravitySlipAngle use this to determine the surface angle.

The magnitude of this vector is typically unimportant as it is automatically normalized, however, if the magnitude is zero then both the _surfaceSlideAngle and _gravitySlipAngle will be ignored resulting in always sliding and slipping. This can be used for moving in a "top down" view where gravity is not used and slipping is always required in all directions.

NOTE: By default, this value is _up.

Additional Resources: Rigidbody2D.Slide, Rigidbody2D.SlideResults