The FROMDEF and TODEF keywords are used to specify which types the relationship comes from and goes to respectively. This is reflected in the diagram and the relationship is drawn from the FROMDEF type to the TODEF type.
When used within a DEFINITION block this is supporting a SINGLE relationship through the definition type. If users need to support more than one relationship through a definition type then a different keyword structure can be used to add them. For a definition of the new structure, see REPRESENTS EXPLICIT RELATIONSHIP.