In VLF-WIN a relationship handler is an RDML function or reusable part that is called to dynamically expand the relationship between a parent and child object. By doing this you can improve filter performance by only adding root or parent objects to the instance list initially.
Relationship handlers are used by the shipped instance list only, they are not applicable when writing your own instance list.
In VLF-ONE a relationship handler is class VF_AC023O snap-in component.
See VLF-ONE Snap-In Points Summary for more details and examples.
This property is in the Business Object Relationships tab.