I think I found something....
from "BMW World"....
A water spot is a mineral deposit that is left behind on the surface of the paint after the water has evaporated. A spot sits on top of the surface of the paint. The deposits may cluster around the edge of the spot, leaving a noticeable ring.
A water stain, on the other hand, is an actual stain that has penetrated into the paint. Some water stains are so severe that they actually need to be wet sanded for removal. The deposits can be difficult to remove if they remain on the car for any length of time.
How to Remove Water Spots & Stains
To remove water spots from a painted surface, use a soft, cotton towel dampened with a little white vinegar. Other products can also be used. A product called Perfect Putty is designed to remove mineral deposit residue.