The caste system is not far from being a slave system. Slavery is a compartment, a stairway in the caste system, except that slaves are at the bottom of the ladder and castes slightly above.
I have descended from slaves. I am emancipated, but I belong to a caste: the caste of emancipated slaves. There is a basic difference between a man who has always been free and a former slave who has been emancipated. These are two different castes.
Back home in Mauritania, one remains emancipated even after several generations. For example, regardless of whether someone was emancipated in 1800, 1900, or 1950, all his or her children will always be emancipated. They will not be free men. Emancipation is a stigma.
Thus, an emancipated individual may not marry an Arab-Berber. That is always a result of the sociological relationship that provides that those who take women are always superior to those who give them. Arab-Berbers may take our women and marry them, but we may not marry their women. That is prohibited by the religious code. We have to stay within the caste.