Some consider a "donor female" anything can is not bred and is being sold, or that can not raise their own calf due to lack of milking ability or bad mothering ability or unable to carry of pregnancy. That is why I wish that more breed associations would offer the ability to look up offspring of a female. When I see "donor potential" listed in a sale catalog, my first thought is "she did not get bred even though we tried", or her natural calves are poor due to milking ability. I ask a lot of questions, and even though I may not get an honest answer, I can usually tell the true story.
I personally consider "donor quality" a female that has at LEAST 4 calves that are of superior quality.
I personally would NEVER flush an unproven female (heifer).
I think probably 60% of the cows being flushed probably shouldn't be. I also think about 80% of the bulls that are being collected and sold as AI bulls shouldn't be either.