This option determines whether the type of all objects are shown in the object list. Together with the objects names you can configure LEX to show any object attributes to be shown directly in the list as additional columns:
There are five important basic information for each object, which can be chosen directly to be shown as a list column or not. The Object Type is one of them:
Each object has one or several object classes assigned, which are derived from the hierarchical class schema of the directory. One of these object classes is the main class which defines the nature of the object and which is sometimes is called 'structural class'. Some directories store an attribute named structuralClass for each object - in other directory environment you can derive the main object class from the order in which the classes are stored in the multi-valued attribute objectClass. LEX tries to evaluate the main class for each object according to the current directory type.
You could configure the Object Type option also by opening the context menu of the object list column header - just right-click on the column header: