If you are doing something like shining a laser beam at the moon, or radar at a planet, then the distances are between the surfaces of the bodies rather than be

If you are doing something like shining a laser beam at the moon, or radar at a planet, then the distances are between the surfaces of the bodies rather than between the gravity centere. And if you are flying a probe towards the sun, the altitude above the surface becomes the more important number than the AU distance.