What is meant by the impossibility "A Buddha can not help someone not related to him"?

What is meant by the impossibility "A Buddha can not help someone not related to him, no relative of him"? Is said to be one of the three things a/the Buddha can not do. How should that be understood? And what is needed to become one that a Buddha could help? Who is a relative, are the relatives, of the Buddha? [Related and given in tiven sphere: [Buddha can not help one not related to him? Buddha kann keinem "Fremden" nicht helfen?](http://sangham.net/index.php/topic,9364.msg19285.html#msg19285) . *(Note that this is not asked for trade, exchange, stacks, entertainment and akusala deeds, but as a share of [merits](http://accesstoinsight.eu/en/dictionary/punnakiriyavatthu) and continue such for release)*