item.xml file in hybris

To define an items.XML file there must be an xsd file which has some rules. In items .XML file there are mainly six tags which has some specific usage and the are written in an order, we must follow that order while declaring items.
The root tag is
<items types></items types>

The sub tags are <atomic types>
<collection types>

<enum types>
<map types>
<relation a type>
 <item types>
Item type is essential one which describes type structure in hybris.  In simple way an item is an instance of type similar to that java, object is an instance of class. 
An itemtype describes how data is placed in structured way based on its XML file say items.XML.