Initialization 
You can use the statement tt:clear to initialize a data root, a variable, or a parameter:
<tt:clear [ref="node"|var="variable"] />
You use ref or var to specify the field that you want to initialize. If no field is specified, the current node is used implicitly. The following fields are possible:
A node node specified according to the addressing rules
During serialization, only variables (or parameters) are initialized. If a node is specified with ref or if the current node is specified implicitly, tt:clear has no effect during serialization.
During deserialization, the specified nodes or variables (or parameters) are initialized.
The transformation below shows an initialization:
Syntax
<tt:transform xmlns:tt="http://www.sap.com/transformation-templates">
<tt:root name="ROOT"/>
<tt:template>
<tt:ref name="ROOT">
<X>
<tt:value/>
</X>
<tt:clear/>
</tt:ref>
</tt:template>
</tt:transform>
During deserialization, the initial value is assigned to the ABAP data object bound to the data root ROOT.