Each line in /etc/subgid 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 group IDs to be that each user may use with the newgidmap command that ordinary users can use to configure gid mapping in a user namespace.
Multiple ranges may be specified per user ID.
subuid(5), login.defs(5), newuidmap(1), newgidmap(1), usermod(8),