When you know which soil layer a find came out of, stratigraphicLayerId records that StratigraphicLayer. This is the most precise vertical provenience an artifact can carry, tying it to the layer's depth, soil texture, and interpretation. The field is optional because not every artifact is assigned to a specific layer. If the layer is later deleted, the link is set to null so the artifact's own record is preserved.
Technical section
Data model, API, and internals — visible to developers.