What are sub-uids and sub-gids that usermod can add or delete?

[habono (Habono Giog)]

-v and -w options of usermod add sub-uids and sub-gids.

What does it achieve?

Can any user be a subordinate user or only some users, and the same about groups?

[dalto]

They are generally used for container applications.

This is a pretty good description:
https://www.funtoo.org/LXD/What_are_subuids_and_subgids%3F