Instead of a field, a mapping can name how the form is opened on the row's nanopublication: to supersede it, making a new version; to derive from it, making a new thing; or to override it, re-editing the same thing under a different key. The form opens filled with the nanopublication's content.

Override is what lets a member correct another member's contribution in place; it works when the row's nanopublication is the one that introduced the thing being edited.