set preferred e-mail address

The database table oc_cards_properties has a field preferred, but it is unclear to me, what it is used for?
There is no setting in the frontend that can select a preferred address.

MariaDB [nextcloud]> describe oc_cards_properties;
+---------------+---------------------+------+-----+---------+----------------+
| Field         | Type                | Null | Key | Default | Extra          |
+---------------+---------------------+------+-----+---------+----------------+
| id            | bigint(20) unsigned | NO   | PRI | NULL    | auto_increment |
| addressbookid | bigint(20)          | NO   | MUL | 0       |                |
| cardid        | bigint(20) unsigned | NO   | MUL | 0       |                |
| name          | varchar(64)         | YES  | MUL | NULL    |                |
| value         | varchar(255)        | YES  | MUL | NULL    |                |
| preferred     | int(11)             | NO   |     | 1       |                |
+---------------+---------------------+------+-----+---------+----------------+

Most contact entries have a 0, but very few rather old entries have a 1 there. Was there a setting in earlier versions?

If there are multiple mail addresses per contact card, it would be awesome to be able to set a preferred address over the others.

Start with the oc_cards_properties table and trace how its preferred field is read or written by the contacts app. The issue names no source files or tests, so first establish the existing behavior and agree on whether the frontend should select a preferred email; done would require that behavior to be specified and covered by validation.