There are 3 system tables that will always be present and that cannot be modified.
Tables
The users table
The users table contains all known users in the system. When a user is created it will automatically be added to this table
The usergroups table
The usergroups table contains all existing usergrous defined in the system. When a usergroup is created it will automatically be added to this table
The projecttemplates table
The projecttemplates table contains all project templates. When a new template is created it will automatically be added to this table.
Restrictions
System tables can not be created, edited, renamed, modified or deleted by a user. The only actions available on system tables are hiding them or creating relations.