just go though below path and put F1 for help window will display it's means.
SPRO> Plant Maintenance and Customer Service >Master Data in Plant Maintenance and Customer Service >Technical Objects >Functional Locations >Create Structure Indicator for Reference Locations/Functional Locations
In standard SAP there is no such mandatory relationship between the two. Very rare you will see Functional location directly in WBS master record.
That relationship is established by specific process and master data design within your particular implementation.
I'd rather suggest you using IW38 report for work orders, where you can filter by both WBS assignment and Functional location.
You may end up with many to many relationship.