Dear engineers, what are the water hummer arestor design/selection basics?

WHA (Water Hammer Arrestor) are usually fittings to suppress the varying shocks in the pressurized piping system. The ideal location is the farthest point or the branch that calls for high flow rate. Zoning (when isolation valves are used to segregate the toilets)  should also be considered while locating the WHA. The best reference is PDI (Plumbing Drainage Institute) where you shall find guidance for selection & having it in the system.