These two pieces of plumbing may be required by your plumber or your municipality. If so, order them ahead of time to save the time and expense.
PL650 Zurn Hair Trap is direct-plumbed into the outflow under your tub. It is rquired by some jurisdictions to prevent debris from entering the city water system. It is not nearly as easy or convenient as the SmartWay Hair trap, but it is the trap required in cases of CODE compliance.
The PL929 Backflow preventer is almost always required to prevent dirty water mixing with clean water in the municipal water supply.