What does these http headers mean?

SEC-FETCH-MODE  navigateSEC-FETCH-SITE  cross-siteSEC-FETCH-USER  ?1UPGRADE-INSECURE-REQUESTS 1

Especially I would like to know what SEC-FETCH-USER ?1 means. Can't understand what the possible values are from here - https://www.w3.org/TR/fetch-metadata/#sec-fetch-user-header

Thanks