Simple transformations are used to serialize ABAP data (or de-serialize XML documents to create ABAP data). Accordingly, it must be possible to address the bound ABAP data in the ST programs.
In an ST program, all data bound to ABAP data objects are considered as data nodes, that is nodes in tree structures, spread from the data roots. To address the ABAP data, you can either access the data roots ort he current node. At any position of an ST program, exactly one node of the tree structure spreading from the data root can be defined as the current node. You can access the data roots or the current node directly and, from that position, access the subnodes.