For outdoor pools, there is no practical reason that MPS (non-chlorine oxidizer) needs to be used. Typically, MPS is used in spas and indoor pools more often because it helps more quickly oxidize residual CC (combined chlorines). In outdoor pools, sunlight does an excellent job of eliminating CCs. MPS can be used to oxidize bather waste in an outdoor pool, but liquid chlorine is much more economical. MPS is not actually an approved sanitizer, like chlorine but can be useful in some situations.