In addition , 6.6.2.8.3 is also specified for certain groups of substances as indicated in Part II and for individual substances as indicated in the Dangerous Goods List of chapter 3 of the Model Regulations based on the decisions of the Committee of Experts . 5.6 Bottom openings : Three possible bottom opening arrangements are proposed , 6.6.2.6.3 ( which indicates three serially mounted means of closure ) , 6.6.2.6.2 ( two serially mounted means of closure ) or N.A . ( Not Allowed ) .