Expert and Installer are two modes to configure an ISP or a PA, accessible through the "Expert Setup" button in the WebUI at startup. There is a different password for each.
Installer can manage every settings : System, Input, Main Speakers, Settings, Presets, Monitoring, Firmware Upgrade and Factory Reset. The Installer mode can enable/disable access to any of these settings to the Expert mode. In this way, Installer can allow a sub-level of configuration to the Expert mode.
By default, Expert only has access to "System", allowing for log export, configuration backup and restore.