It foes immediately after the toilet
Although a vented loop is traditionally considered necessary only on hoses connected to below-waterline thru-hulls, we strongly recommend installing 'em on sailboats even if the head only discharges into a holding tank. Due to heeling, sewage can run back toward the toilet from the tank. No...it won't sink your boat...but without a siphon break, the only standing between you and an overflowed bowl is the joker valve in the head...which nobody ever seems to think needs replacing until a bowl does overflow. So IMHO, it's beter to be safe and install a vented loop in the head discharge hose than to find yourself mopping up half a holding tank.
