When you need filters to flow in both directions, but use sparingly as it can cause ambiguity

Only for many-to-many relationships, although related to schema design, it does not describe the cardinality or relationship direction behavior asked in this item.

Always, for maximum flexibility, although related to schema design, it does not describe the cardinality or relationship direction behavior asked in this item.

Never, it's not recommended, however, this confuses data modeling structure with report-layer behavior and therefore misses the requested concept.

A schema decision that improves usability in the Fields pane but does not implement the required relational behavior