 A toilet is the main connection, with the option of a right or left-hand outlet to the 3" inlet with a choice of 1-1/2" or 2" in size.It is used for keeping stack-vented fixtures high to the joist space and thus conserving head room in a basement.As the water closet must be the lowest fixture, the smaller side outlet (usually used to connect the bathtub trap arm) enters slightly above the larger connection.