Serialization and Deserialization 
Serialization and deserialization is primarily based on following schema:
During serialization, the literal elements of templates are passed to the target XML document unchanged; the ST commands specified in between are executed.
During deserialization, the source XML document is treated as an inbound stream and compared to the template element by element. Elements (or attributes or texts) of the inbound stream with the same name are consumed if they are positioned in the ST program at the same place or if a command is positioned there that can process the element. After the successful comparison or consumption of the element, the comparison of the subsequent element starts.
When you define a Simple Transformation, pay attention to whether serialization and deserialization are symmetric or not; see: Symmetry of Serialization and Deserialization.
During serialization and deserialization, the XML documents are processed in the form of write and read streams. When ABAP methods are called from Simple Transformations, there is an option to edit these XML streams; see: Access to the XML Streams in Methods.
Note
Serializations and deserializations are subject to the general restriction that the content of a data root cannot be modified by serialization; only write access is possible to a data root during deserialization.