Array of sub-properties. This is used in the core for the "sys_file" table:
$GLOBALS['TCA']['sys_file'] = [ 'ctrl' => [ 'security' => [ 'ignoreWebMountRestriction' => true, 'ignoreRootLevelRestriction' => true, ], ... ], ];
Allows users to access records that are not in their defined web-mount, thus bypassing this restriction.
Allows non-admin users to access records that on the root-level (page-id 0), thus bypassing this usual restriction.