Is core flag
From Joomla! Documentation
The "Is core" flag, or 'iscore', is used to protect extensions from being uninstalled in the user interface. This check is manifested in different ways with the primary method being a database field called 'iscore' in tables, such as the plugins table. This flag exists in Joomla! 1.5 or lower with Joomla! 1.6 moving to use the field 'protected' in the extensions table instead to control if an extension may be installed or uninstalled.