This is not documented (yet). The best information is found in the code:
/**
* Enrol user into course via enrol instance.
*
* @param stdClass $instance
* @param int $userid
* @param int $roleid optional role id
* @param int $timestart 0 means unknown
* @param int $timeend 0 means forever
* @param int $status default to ENROL_USER_ACTIVE for new enrolments, no change by default in updates
* @param bool $recovergrades restore grade history
* @return void
*/
public function enrol_user(stdClass $instance, $userid, $roleid = null, $timestart = 0, $timeend = 0, $status = null, $recovergrades = null) { ....