Complex sentence: Those systems in which there is little to no mobility, even on an intergenerational basis, are considered closed stratification systems, such as caste systems in which one's position at birth is the social position one holds for a lifetime.
Split into simpler sentences: Those systems in which there is little to no mobility, even on an intergenerational basis, are considered closed stratification systems. For example, in caste systems, one's social position at birth is the position one holds for a lifetime.