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