Y valve number one diverts the output of the toilet either directly overboard or to the holding tank
Y valve number two allows either the holding tank or the head to be pumped overboard (separate holding tank pump or toilet pump as in above)
Y valve number three selects between a pumpout station or using the holding tank pump (Any non-corrosive 1 1/2" pump) (optional; I have never used a pumpout station)
I have two heads that have both used this system for 24 seasons
Avalon, Santa Barbara, etc do not require securing the valves-they use dye instead (green in Avalon, red in Santa Barbara, others ???)