HI
A navigation path is an object that defines the drill path used in SAP Business Objects reporting tools. A drill path is a list of drillable business objects that allow a report analyst to drill down on a dimension. A navigation path object can be one of two types: default or custom.
If you want to create custom hierarchy From within a Business Layer (*.blx), click Navigation Paths in the IDT, then you can use the navigation path.
http://scn.sap.com/docs/DOC-22087
http://events.asug.com/2012BOUC/1202_Information_Design_Tool_Tips_Tricks_and_Workflow.pdf