The key is used to enter the link object, in the form of an expression (or a group of expressions separated by a semi-colon if the link key is composed of several parts). This is used to identify the record (of the link object) from which the link will start.
Warning there are limits to the definition of these automatic links. In fact, it can only be used on link expressions for fields in the principal fields. In view of the very simple structure of the link table, it is always possible to fill this table using a personalisation.