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This is the description from the PHP Docs: @param array $logextra the format is standardised by logging API

The only reference to extra information in the Logging API docs seems to be: 

The log store plugin(s) stores the event data optionally including extra information such as remote IP or original logged-in-as user id

The standard logstore uses it like this:

public function get_events_select($selectwhere, array $params, $sort, $limitfrom, $limitnum) {
        global $DB;

        $sort = self::tweak_sort_by_id($sort);

        $events = array();
        $records = $DB->get_records_select('logstore_standard_log', $selectwhere, $params, $sort, '*', $limitfrom, $limitnum);

        foreach ($records as $data) {
            $extra = array('origin' => $data->origin, 'ip' => $data->ip, 'realuserid' => $data->realuserid);
            $data = (array)$data;
            $id = $data['id'];
            $data['other'] = unserialize($data['other']);
            if ($data['other'] === false) {
                $data['other'] = array();
            }
            unset($data['origin']);
            unset($data['ip']);
            unset($data['realuserid']);
            unset($data['id']);

            $event = \core\event\base::restore($data, $extra);
            // Add event to list if it's valid.
            if ($event) {
                $events[$id] = $event;
            }
        }

        return $events;
    }