Hi there!
Welcome to the forum.
As you may know, in different areas of the world, we have two different areas of buoyage. A, and B.
This is shown on the map below:
To explain this, take a look at Ballast's topic, he's at sea at the moment, but his topic is there for you to look at http://www.shipsim.com/ShipSimForum/index.php/topic,13434.0.html
If you have any more questions, don't hesitate in asking!
Hope to see you around!
Kind Regards,
Jack.