One part of the MySQL installation process is data directory initialization (see Section 18.104.22.168, “Initializing the Data Directory Manually Using mysqld”). During data directory initialization, MySQL creates user accounts that should be considered reserved:
'root'@'localhost: Used for administrative purposes. This account has all privileges and can perform any operation.
Strictly speaking, this account name is not reserved, in the sense that some installations rename the
rootaccount to something else to avoid exposing a highly privileged account with a well-known name.
'mysql.sys'@'localhost': Used as the
sysschema objects. Use of the
mysql.sysaccount avoids problems that occur if a DBA renames or removes the
rootaccount. 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.