"--" is shown in the Category, Allergens and Dislikes columns when the value is empty and either you are viewing Deleted items or you lack the Edit Ingredients permission (so inline editing is off). With edit rights on active items, an empty cell in these three columns is left blank and clickable so you can set the value. The Supplier column behaves differently: it shows a blank cell (not "--") when no supplier is set, regardless of permission, and is only clickable to edit when you have permission and are viewing active (non-deleted) items.