fox used to make seperation between devices on substrate. but the hight of this fox will be apprx. 6000a . this much hight can not eatch. so to avoid to grow the fox on the active area , active area can be defined by nitride. fox can not grow on nitride and there will be a small hight of nitride . after growing fox nitride can eatch easily.
and that area will define the active area for nmos and pmos.