MySQL 9.5 Reference Manual Including MySQL NDB Cluster 9.5
One part of the MySQL installation process is data directory initialization (see Section 2.9.1, “Initializing the Data Directory”). During data directory initialization, MySQL creates user accounts that should be considered reserved:
'root'@'localhost: Used for administrative
purposes. This account has all privileges, is a system
account, and can perform any operation.
Strictly speaking, this account name is not reserved, in the
sense that some installations rename the
root account to something else to avoid
exposing a highly privileged account with a well-known name.
'mysql.sys'@'localhost': Used as the
DEFINER for
sys schema objects. Use of the
mysql.sys account avoids problems that
occur if a DBA renames or removes the root
account. This account is locked so that it cannot be used for
client connections.
'mysql.session'@'localhost': Used
internally by plugins to access the server. This account is
locked so that it cannot be used for client connections. The
account is a system account.
'mysql.infoschema'@'localhost': Used as the
DEFINER for
INFORMATION_SCHEMA views. Use of
the mysql.infoschema account avoids
problems that occur if a DBA renames or removes the root
account. This account is locked so that it cannot be used for
client connections.