Some tanks use a Y valve, one side for overboard and the other for the Holding tank. On this type you have to vacuum or pump out the waste.
On the other type the waste fills through the top of the tank and has a hose on the bottom which connects to a through hull fitting, when you open the through hull fitting the waste goes overboard. If it clogs you can pump it out from the deck fitting.
I’m sure the BVI government can come up with an easier way to the destroy the charter industry