When different social differences overlap, one particular social difference gains in prominence. This leads to the creation of social division and tension. For example, in Northern Ireland, the social differences of class and religion overlap each other. The catholics are usually the ones who are poor, while the protestants are the ones who are well off. The religious differences are accentuated by this overlap. As a result, there are conflicts between the two religious groups.
In other words, a social difference becomes a social division when it coincides with other social differences, which in turn serve to heighten the said difference.