It is a subjective vote, as opposed to the title for leading sire in North America that looks strictly at the earnings of the stallion's progeny in the given year.
Currently, a mare is eligible for consideration if one particular offspring, conceived and foaled in Kentucky, won a Grade I stakes race in the award year.
Consideration may then also be given to previous foals, specifically the number of stakes winners produced and their earnings.
[1] The rules were more relaxed in the past, essentially requiring, as one bloodstock expert observed, only that "the mare be boarded in Kentucky.
"[2] Mares who have produced multiple graded stakes winners may also be given the informal title of "blue hen".