An operating system account you use to run Configuration Manager.
If you are upgrading on the same machine where Content Platform Engine (or Content Engine) was previously installed, use the same Configuration Manager user account that you originally used during installation. The program requires this to detect that it is an upgrade and to use the same installation values.
If you are upgrading on a new machine, where Content Platform Engine (or Content Engine) has never been installed, create config_mgr_user, as explained here.