Each line in /etc/subuid contains a user id and a range of suboridinate user ids that user is allowed to use. This is specified with three fields delimited by colons (":"). These fields are:
This file specifies the user IDs to be that each user may use with the newuidmap command that ordinary users can use to configure uid mapping in a user namespace.
Multiple ranges may be specified per user ID.
subgid(5), login.defs(5), newuidmap(1), newgidmap(1), usermod(8),