The excavationUnitId field is one half of the link a FeatureUnit represents, pointing to the ExcavationUnit side of the connection between a feature and a unit. Together with featureId it forms the record's composite identity, so each unit-to-feature pairing is stored just once. This field is what lets the system list every feature exposed within a given excavation unit. If the referenced ExcavationUnit is deleted, its FeatureUnit links cascade away, and the field is separately indexed to make unit-side lookups fast.
Technical section
Data model, API, and internals — visible to developers.