the technical difference between OU(D) ice cream and Chalav Yisroel Ice Cream?
The OU is not claiming to have supervised the milking. “Chalav Yisroel” ice cream would only receive that designation if the milking was supervised and that milk tracked until the ice cream was packaged (unless it was under Feif Un’s hechsher ? ).
I don’t know if that’s true, but even if it is, if you would only eat CY you wouldn’t know that (even if it were true), and you would be happy with what you have (i.e. CY) since you wouldn’t know any better.
Unless you used to eat C”S.