'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.