The featureId field is one half of the link a FeatureUnit represents, pointing to the Feature side of the connection between a feature and an excavation unit. Together with excavationUnitId it forms the record's composite identity, so each feature-to-unit pairing is stored exactly once. This field is what lets the system gather all the excavation units a single feature spans. If the referenced Feature is deleted, its FeatureUnit links cascade away with it, keeping the spatial associations consistent.
Technical section
Data model, API, and internals — visible to developers.