Analyzing the Transfer Structure 
So that data from your legacy system can be transferred automatically into the R/3 System, it should be delivered in a particular format. Therefore you should know the exact transfer structure so that you can convert your legacy data in a later step to the appropriate format.
The Data Transfer Workbench supports this step with two different display options:
The system displays a complete data records of the transfer file with its structure tables. You can see the fields of a structure and the length and sequence in which they should be in the initial transfer file, as well as the object-specific NODATA value. You can display the fields of the structure by double-clicking on a line in the structure.
The system displays the structural setup of the transfer file. The interface structure shows a technical description of the transfer file and displays the structures and fields that may be used, and their sequence.
Procedure
You can print the interface structure (by choosing List ® Print). You can use the printout for subsequent steps, such as assigning fields.