A field of a collection: its name, its type (Cardbox.Type) and
that type's options.
key is made from the name when the field is added ("Due date" is
due_date) and never changes: a record keeps its values under their
fields' keys, and a saved view names fields by key, so a field renamed
is renamed everywhere at once. Queries and writes take a field's key or
its name.
Summary
Functions
The field's choices, for a select, a multi-select or a status; [] for others.