F_SHELF is an optional table, typically implemented as a child of F_RACK. At some fiber sites, a rack has slide-out shelves that house a stack of devices or splices. If not implemented, then patch panels and devices are typically direct children of racks, with no intermediary shelves.
You can find existing relationships and create new ones in ArcCatalog. Almost all relationships are found at the geodatabase level, as opposed to the dataset level. Below is an image of the final properties for this relationship. For more information about these settings, see the relationship topic Summary of Recommendations.