The divider wall comes with all Modpools 16' or longer, which include the Spa Seating Feature. The divider wall is a honeycomb structure, allowing water to pass through it while also acting as an insulator to retain heat on one side. It can't be used as a watertight wall, and the whole pool must be filled with water for it to function.
The divider is removed by sliding it out of the water; it can get quite heavy because it fills up with liquid. We usually recommend it be done in teams of two or by pushing upward from within the pool, going slowly to let water drain out.