Defining Structure Relationships
You establish the structure relationships between the project and the SAP system.
The source structures must have been defined.
From the selection list of steps, choose Maintain Structure
Relations.
A screen appears where the structures used in the SAP system are displayed.
The system shows you which target structures must be filled.
If you want to know which fields are each associated with an SAP structure:
Maintain structure relationships:

Many batch input and direct input programs use a control record called BGR00 or BI000. Always assign the source structure of the top hierarchy level (the header structure) to this record.

Multiple SAP structures can be assigned to a source structure, however, only one source structure can be assigned to an SAP structure. If you want to assign multiple source structures to a target structure, proceed as follows: Create your source structures in the conventional way. Then assign the lower level source structure to the target structure. You can then use the fields from both source structures for the fields of the target structure.
If you want to change a structure relationship, you have to delete the existing structure relationship first.
See Deleting Structure Relationships