The deck pumpout fitting and the overboard through-hull valve are normally pressure tight and will function alternatively as selected. Overboard discharge options both before and after the holding tank. Flexibility is offset by complexity. Overboard discharge option before the holding tank. When using a type I or II holding system, you should install a holding tank for use when boating in environmentally sensitive areas or when moored or dockside.
External pump is required to empty holding tank.
The section on equipment selection addresses many of the options available.