The subordinate gid file
Each line in /etc/subgid contains a user name and a range of subordinate group ids that user is allowed to use. This is specified with three fields delimited by colons (“:”). These fields are:
numerical subordinate group ID
numerical subordinate group ID count
This file specifies the group IDs that ordinary users can use, with the newgidmap command, to configure gid mapping in a user namespace.
Multiple ranges may be specified per user.
Per user subordinate group IDs.
Backup file for /etc/subgid.